The philosophy of nature contemplates truths about the cosmos, that is, the physical universe. More precisely, it is that part of speculative philosophy which studies changeable being in light of the principles and causes of changeable being. Since, as Heraclitus said, “nature loves to hide,” we need natural philosophy to understand more fully our place in nature and to seek out its truths. The founders of natural philosophy include ancient Greek thinkers such as Aristotle, and among its cultivators stand medieval giants such as St. Albert the Great. The heirs of natural philosophy are alive and well today. Through natural philosophy, we embark upon natural inquiry properly speaking. Natural philosophy fires the intellectual workshops of the various natural sciences and informs human action in our practical reasoning and in the technical and fine arts. It also shepherds us to the threshold of contemplation of the higher realms of metaphysics. Because it considers such a cosmic range of things, a certain “fullness of contemplation” belongs to natural philosophy (St. Thomas Aquinas, Commentary on John, Prologue, n. 9).{1}

Historically, “philosophy of nature” has been given a variety of definitions and approaches. Originally, it was known as “physics” because it studied physical things (physica), or natural things (from the Greek physis or Latin natura meaning “nature”). The ancient Stoics or Epicureans articulated materialistic doctrines in “physics,” especially as these contributed to their ethical and quasi-theological views. The Platonists, however, eschewed the domain of physics, considering the realm of nature subject to opinion but not to knowledge. Other “natural philosophers” inquired into nature’s secrets for the purpose of controlling its hidden powers; these included medieval figures not unrelated to the initial stages of early modern science. Even Boyle and Newton were called “philosophers of nature,” since the very word “scientist” did not exist until the 19th century.{2} As the natural sciences advanced and became bound up with technical mathematics and engineering prowess, other thinkers sought to return to the “philosophy of nature” in more holistic, poetic, or even mystical senses. Our consideration of natural philosophy follows the Aristotelian-Thomistic tradition, part of the perennial philosophy which, in the providential course of history, has found a home in the Catholic Church.

Despite this ancient and venerable lineage, our contemporaries raise a challenge that must be noted at the outset: Insofar as the philosophy of nature relies upon ideas elaborated during the infancy of the natural sciences—as flourishing as such ancient Greek or medieval Arabic and Latin periods were in other respects—it is unsound and outdated today. The natural sciences have entered into a more responsible and mature phase. This is to say nothing of the fruitless or confused character of many of the concepts found in natural philosophy which leads to outright contradictions with modern scientific discoveries. If any philosophy of nature is to be gleaned from the discoveries of the modern sciences, then it is a materialistic one bereft of the features that once made archaic natural philosophies plausible or attractive.

In response, consider the possibility that the philosophy of nature relies upon principles which were present at the infancy of the natural sciences, but that these principles are perennial ones, truths not merely old but foundational. Perhaps various concepts or arguments (now outdated), which were present at a certain historical stage of natural philosophy, are no longer essential to the inquiry it proposes. Such a perennial philosophy of nature would be a tradition of inquiry able to dialogue with the advances of the modern natural sciences even as it sheds light on their findings. What Pope Leo XIII wrote long ago is still true: “Between certain and accepted conclusions of modern physics and the philosophic principles of the schools there is no conflict worthy of the name” (Aeterni Patris, no. 30). The philosophy of nature is part of “our heritage of knowledge and wisdom” that “has indeed been enriched in different fields” (John Paul II, Fides et ratio, no. 91).

To understand the philosophy of nature more fully, we will consider what it studies and how (Section I), along with its principal ideas and insights as a true philosophy of the cosmos (Sections II–V). By way of conclusion, we will consider its relationships to the other parts of philosophy as well as the other areas of human knowledge (Section VI), especially Catholic theology (Section VII).

I. Defining the Philosophy of Nature

What does the philosophy of nature study? How does it arrive at its goal? Indeed, what is the goal of natural philosophy? After considering these questions, we will describe the parts of natural philosophy.

Subject

The reality of motion is ubiquitous in our daily experience. Not only do we change, but our environment and the things in it change as well. The task of natural philosophy is to study changeable or mobile being (ens mobile). This is its broad conceptual horizon or subject matter.

Motion includes change of place but also qualitative changes such as heating and cooling, growth and maturation, and conception and death. Natural philosophy studies everything from the dog walking in its peculiar fashion, the dog warming itself in the window, the sun warming the dog, the dog digesting food and maturing, to the dog gestating its young. Natural philosophy, like philosophy as a whole, begins with wonder and desire: We wonder about and desire to know changeable beings in their very changes. Why is the dog warming itself? How is the sun warming it?

Perhaps one might say instead that natural philosophy is about empirical or sensible things. However, since they name ways we know things, but not the things we wish to know, these terms offer too indirect a description. It is the changing things themselves, after all, which we observe and desire to know. Indeed, some philosophers admit to empirical or sensible reality but deny that motion is real or knowable. Thus, by naming mobile being as our subject, we describe the end of natural philosophical inquiry more adequately and avoid the suggestion that some topics are out of bounds which should be included, such as things not directly detectable by the senses (e.g., the human soul).

Is it better to say, as many do, that natural philosophy is about material things? While this is a form of unity by agreement on a common description, it is not helpful in the long run, for the terms “matter” and “material” hide many equivocal meanings. Analogously, to define theology as “about divine things” would be agreeable to a polytheist and a Catholic; and to define psychology as “the study of consciousness” would be agreeable to a panpsychist and—to him—would also encompass chemistry. Motion as an effect catches the mind’s eye, thus demanding explanation. This is the basis upon which we characterize the whole field of inquiry. Materiality is on the side of a cause or reason why things move, and it is not the only such cause. The sun warms the dog because of a certain material composition (of the dog and of the sun), but that is not the only reason why.

Perhaps we could say that natural philosophy studies physical or natural things, meaning those things which have a nature (natura in Latin, or physis in Greek). However, this proposal suffers from a weakness similar to the previous one. What is a nature? If a dog suns itself because it is in its nature to do so, then we are again naming the subject based upon a cause rather than through a description apt to include the entire ambit of our subject. The inquiry of the natural philosopher is about more than just the causes or reasons which satisfy our initial wonder; analogously, criminologists study more than the perpetrators of crimes.

Finally, it is not sufficiently accurate to say that natural philosophy studies real things or beings. The reason for our wonder about such things is that we see them change. Look at that thing there, moving about! Why does it do that? To ask about its very reality or its very being may be a different question. Indeed, what if being is itself a sort of motion or change? We would have to determine the difference between existing and moving first, and this would require investigation of what it is to move. Indeed, this problem of the being of change is precisely the first question that natural philosophy encounters. Besides, defining the subject of the philosophy of nature as the study of being would conflate it with metaphysics.

St. Thomas argues that natural philosophy is about mobile being (see Commentary on Aristotle’s Physics, bk. I, lect. 1, nn. 1–4).{3} Mobile beings act and move as they do in virtue of their natures, and so the term “mobile” as precisely qualifying “being” more aptly and adequately captures the ambit of our inquiry as natural philosophers, describes the object of our wonder, and indicates what we hope to know. Even a description such as “mobile body” is too narrow and too derivative, since, as Aristotle argues in Physics, quantity is a necessary condition of mobility and thus is an object of explanation in natural philosophy and not its subject.{4} To say that we inquire about mobile beings has the added advantage of including the strengths of the alternatives noted above, insofar as mobile beings are also sensible or empirical, material, natural, and real.

Where to Begin?

If we seek to know mobile beings, then how should we go about doing this? In part, the answer depends upon the goal of our inquiry. It also depends upon the methods required to know mobile beings at all.

The goal of natural philosophy is to know why changing things act or move or behave as they do through insight into their principles and causes. This is the object of its inquiry. A firm grasp upon the causes of a thing is what Aristotle describes as a science. “For we do not think that we know a thing until we know its first causes and first principles and have carried our analysis as far as its elements” (Aristotle, Physics, I.1). Why is the dog warming itself? Does it have a certain temperature as a goal, or is it seeking comfort and rest? Why does it have such “warming” behavior in the first place? Why can’t it keep itself warm enough in some other way? To know the answers to such questions is to have, in various ways, causal knowledge about canine nature.

If scientific or causal knowledge of mobile beings is the goal, then by what means should we proceed? Indeed, how should we begin?

Aristotle found the proper way to begin the study of nature.{5} Rather than following the method of Descartes, starting with what is conceptually clearest and most imaginatively distinct to build hypothetical-deductive models of nature, Aristotle tells us two things. First, we must draw from sense experience of the natural world itself. Indeed, he practiced what he preached and studied myriads of living things on the island of Lesbos. Still, such experience is limited in many ways. Our physical lifespans and location limit it, as do our natural senses unaided by instruments (telescopes, microscopes, etc.). Another limitation is that we could notice something through our senses but fail to intelligently grasp what it is or why it is happening. (Why does the sun move through the sky? Why is the sky dark at night? Why does water expand when it freezes?)

An empirical foundation is the basis of conceptual insight and inductive argument, “for though the act of sense-perception is of the particular, its content is universal” (Posterior Analytics, II.19). The universal concepts which are the seeds of the intellectual life, including natural philosophic inquiry, are gleaned from sense experience. Therefore, and second, we must acquire not just sensible or empirical “data” but intellectual insight: “The natural way of doing this is to start from the things that are more knowable and certain to us and proceed toward those that are clearer and more knowable by nature” (Physics, I.1). To proceed in this way is to begin with concepts which are more vague or general than they are distinct and specific. We understand what something is in general more readily and with more certitude than we understand specific concepts because the conceptual “entry fee” is easier to pay. Our minds begin in a state of ignorance, and we must build up our conceptual insights gradually. For instance, we cannot understand how to define a triangle without previously understanding other concepts (such as lines, figures, or the number three). The conceptual content (the comprehension of a concept) is more intensive for “triangle” than it is for a more general concept, such as “shape.” Thus, accurately using the term “shape” with understanding is easier than understanding what distinguishes a circle from an oval or an ellipse. We understand what is more general before we understand the more specific.

One of Aristotle’s examples is that “children begin by calling all men ‘fathers’ and all women ‘mothers,’ but later on distinguish each of them” (Physics, I.1). At a certain age, children will see an adult woman and ask, “Where are your kids?,” thinking that every adult woman is also a mother. This is incorrect, in part, because of an inability to conceptually distinguish the two cases by referring to relevant experiences. However, such children are, in part, correct because they can identify “adult woman” in general.

Natural philosophy begins in this conceptual landscape of what is close to common human experience of nature. However, we must heed the warning implicit in Aristotle’s example as much as we take heart from the hopes it indicates. Progress is not without its risks or rewards:

For it is because of wonder that men both now and formerly began to philosophize about less important matters, and then progressing little by little, they raised questions about more important ones, such as the phases of the moon and the courses of the sun and the stars and the generation of the universe. (Metaphysics, I.2)

Clarifying the Goal, Method, and Parts of the Philosophy of Nature

Having described where to begin, let us circle back to the goal of the philosophy of nature, clarifying that goal by comparing and contrasting the philosophy of nature with the natural sciences.

The goal of the philosophy of nature is to know the principles and causes of changing things. A principle is “that which is primary, whether the existence of a posterior follows from it or not,” while a cause “is said primarily only of that from which the existence of the posterior follows. Hence we say that a cause is that from whose existence another follows” (St. Thomas, On the Principles of Nature, ch. 3). For example, the principles of mobile beings include the starting place of a motion as well as the source of why something moves from that spot. The causes of changeable things—and all causes are in some way principles—include Aristotle’s famous four causes: matter, form, agency, and finality. Material causes can also be called the elements of a mobile being. When we study natural philosophy, we seek to know these principles, causes, and elements in as much specificity and concrete detail as possible.

To achieve this goal, we have already noted that the method of natural philosophy is to begin with common experience, the insights into nature which are commonly available, unavoidable, and implicit in any such experience (e.g., that things move or that some things are able to change other things). Intellectually, our inquiry proceeds along two paths: the order of determination and the order of demonstration.{6} The order of determination goes from a consideration of what is more general to what is more specific. For example, the natural philosopher defines motion in general before studying local motion, and considers this before studying the motions peculiar to stars or to salmon. Proceeding in this fashion is not merely for the sake of efficiency and to avoid repetition (see Aristotle, On the Parts of Animals, I.1). This method of progressive determination also allows us to gradually clarify our understanding based upon what is more certain and easier to know, thus avoiding error when grappling with what is less certain and more difficult to know.

By contrast, the order of demonstration is between causes and effects. We still begin with what is better known to us at first. These are usually effects, clues or signs of their causes. Seeing the cause as the cause produces a new manner of understanding the effect. For instance, it is easier to know that certain physical things are alive, but it is more difficult to define life in terms of its causes and to give an account of why dogs and dolphins are alive but diamonds are not. The natural philosopher investigates the cosmos by coordinating both the orders of determination and demonstration in his research.{7}

What about the parts of the philosophy of nature? Historically, natural philosophy embraced many parts within its inquiry. Aristotle and St. Thomas divide the philosophy of nature according to the various kinds of motion.{8} Thus, one part of the philosophy of nature studies motion in general along with its conditions, causes, and properties; other parts consider change in place, change in qualities, and then change in size (growth) as well as other motions associated with living things. This yields the following outline:

A. The study of motion in general (Aristotle, Physics)

B. The study of motion specifically
   B.1. Locomotion (Aristotle, On the Heavens)
   B.2. Alteration (Aristotle, On Generation and Corruption and other works such as his Meteorology)
   B.3. Growth and other motions in living things (Aristotle, On the Soul and other biological works)

Historically, this Aristotelian division corresponds to a slow descent through the geocentric cosmos, from (A) all moving things in general to (B.1) the local motion of heavenly spheres and elemental bodies to (B.2) the alterations of sub-lunar realms of mutable inorganic things and (B.3) the motions of living things. However, the philosophy of nature is not trapped in this historical context.

It might seem that this description of “philosophy of nature” overlaps too much with the natural sciences. What is the relationship between the philosophy of nature and the various natural sciences (astrophysics, genetics, etc.)?{9} Are these different sorts of inquiries? If so, how are they different? Are they completely unrelated? Furthermore, does the philosophy of nature have a source of evidence for its inquiries by which it differs from the various natural sciences?

For now, the former questions we set aside, but the last question can be answered based upon points already established above. The philosophy of nature begins with common experience, while the various natural sciences begin with much more particular experiences (and experiments) of the natural course of things.{10} For example, common experience familiarizes us with a wide range of motions in general (e.g., heavy things fall). However, specific and technical experiments are needed to mount an argument to reveal a particular principle of change like the law of accelerated fall (that a body falling due to gravity increases its velocity as the square of the time). Still, there is a certain continuity between the two. The more commonly available experiences of nature (heavy things fall) are the basis for the narrow, more technical ones (precisely at what rate heavy things fall). The most common and general conceptions of changing things (such as an idea of change itself; that change takes time; that some changes are faster than others; or that there are different kinds of things that change), must be carefully developed by making distinctions and enriching our conceptual repertoire with further experience. Some of these experiences can be highly specialized and even technologically enhanced (such as measuring time with a watch; calculating which star is moving the fastest in a given cluster; or distinguishing one chemical reaction from another). Nonetheless, true insights in the natural sciences build upon the more general ones with which the philosophy of nature begins.

It therefore seems to be the case that the contemporary sciences carry on the lines of inquiry in (B) under more modern guises: physics (B.1), chemistry (B.2), or biology and related studies (B.3). Each area still studies things insofar as they change: a physicist studies mechanical movements in terms of forces and energy; a chemist studies reactions in terms of electron exchange; and a biologist studies the variation of traits through generations of a population in terms of genetics. Yet such contemporary investigations do not have a general inquiry corresponding to Aristotle’s in (A). Thus, one might conclude that the philosophy of nature is distinguished from the contemporary natural sciences by a difference in generality from the greater (A) to the more specific (B).

Be that as it may (or may not) be, the philosophy of nature begins

with what is most common to all natural things, namely movement and principles of movement, and from there [proceeds] by way of concretion or application of common principles, to determinate mobile things, some of which are living bodies. (St. Thomas, On Sense and What is Sensed, prol., n. 2).

This order of determination and demonstration is an intellectual order of inquiry that benefits from the ever-increasing riches provided by natural history and experimentation. At the very least, what we should say is that the philosophy of nature (restricted to (A) for the present) dialogues with the specific natural sciences in their contemporary versions as given under (B). In this way, one might consider the philosophy of nature to be a sort of philosophy of the sciences (a philosophy of physics, of chemistry, of biology, etc.).

Finally, for the sake of a thorough explanation, we should take note that there are other, ancillary parts of the inquiry of natural philosophy by way of preparation. For instance, the liberal arts, especially logic and mathematics, aid in the proper acquisition of the natural philosophical habit of contemplation of the cosmos. Also, personal experience with changing, natural things and their environments or habitats—a personally acquired natural history of sorts—is a condition as well. Without it, the concepts and principles of the philosophy of nature will not be really apprehended but instead grasped in a merely nominal or purely notional way.

II. The Principles of Change

Having defined the philosophy of nature, its goal, and its methods, we will now consider the main waypoints of insight along its path of inquiry. The philosophy of nature treats of the principles of change (this section), the natures of physical substances and their causes (Section III), motion and its properties in various domains of the cosmos (Section IV), and the ultimate causes of motion (Section V).

Recall that the natural path of inquiry requires that we begin with general considerations. Thus, we first ask in general about the necessary conditions for something to change. The philosophy of nature actually begins with an aporia or difficulty (more bluntly, a “roadblock”): a challenge to the very intelligibility of change.{11} This is a logical question to raise at the outset of trying to understand change. In order to address this question, it is necessary to formulate the problem of change and its solution, consider the principles of change, and then reflect upon the importance of these initial insights in natural philosophy.

The Problem of Change

The problem of change was first formulated by the Greek philosopher Parmenides.{12} His argument, found in the fragmentary remains of his great philosophical poem, can be paraphrased as follows:

1. Every changing being is what comes either from being or from non-being.
2. No being is that which comes from either being or from non-being. 

Therefore, no being is a changing being.

Premise (1) is proposed as an exclusive disjunct. If some thing did not exist previously but now does, then what was its status before? Being or non-being seem to be the only options. Premise (2) adds the claim that neither option will work. If the dog which comes to be already existed before, then it came to be from being, which seems impossible, as it already exists. However, if the dog did not exist, then it cannot come to be from non-being, since nothing comes from nothing. As Parmenides states, “It is right both to say and to think that what-is is: for it can be, but nothing is not” (Curd ed., DK B6). His conclusion can also be stated as “all being is unchanging being.”

Parmenides highlights the importance of such principles as non-contradiction and identity but goes too far. A thing cannot both be and not be at the same time and in the same way. A thing is the same as itself, but this does not prevent it from changing. However, in order to avoid Parmenides’s paradoxical argument, we will need a distinction.

Heraclitus also proposes a problem of change in a formulation opposite to that of Parmenides:

3. Every unchanging being is a being that is the same as it was.
4. No being is a being that is the same as it was. 

Therefore, no being is an unchanging being.

Or, the conclusion could be put as follows: “All being is changing being.” This is the thinking behind Heraclitus’s famous dictum that one cannot step twice into the same river, for “it” is constantly changing. Premise (3) states a truth about the condition of being unchanged which premise (4) denies to things in our experience. It appears to our senses that everything—whether quickly or slowly—moves and changes. Even time itself seems to include all things in a sort of flux. Thus, for a being to remain unchanged, it would have to exist apart from time.

Heraclitus emphasizes the importance of our sense experience of physical things, which change in time, but goes too far and undermines our ability to reason about them.{13} A thing can both change and remain the same (e.g., a boy grows up to be a man, remaining the same person). Like Parmenides, Heraclitus proposes an argument which undermines our intellectual grasp of what is so evident to our senses.

In the wake of such arguments, the ancient Greek materialists also oppose our sense experience of physical things to our reasoning about them. Considering that Parmenides was right, that between being and non-being there is no third, but also thinking that Heraclitus was right to defend the ubiquity of change, their theories tend to posit a certain dualism. On the one hand, our experience and opinions assert the reality of change. On the other hand, however, that things come into being and pass away is only an appearance. In truth, what really exists are certain fundamental, unchanging principles.

Consider Empedocles, for example. His principles are the four elements, which are mixed and unmixed: “There is coming-to-be of not a single one of all mortal things, nor is there any end of destructive death, but only mixture, and separation of what is mixed, and nature [physis] is the name given to them by humans” (Curd ed., DK B8). Our names for the species and wholes apparent to sense are mere names; indeed, according to Empedocles, we are “… Fools. For their thoughts are not far-reaching—those who expect that there comes to be what previously was not, or that anything perishes and is completely destroyed” (DK B11).

Or, consider Democritus, whose atomism is still relevant to the philosophy of nature’s debates on such a fundamental point. The atoms and void are unchanging principles underlying all that we sense and opine about: “In reality we know nothing about anything, but for each person opinion is a reshaping [of the soul-atoms by the atoms entering from without]” (DK B7). Just like Empedocles, Democritus opposes our senses to our reason. For instance, sensible qualities exist by human convention: sweet and bitter or hot and cold or even “by convention, color; but in reality, atoms and void” (DK B9). Rather, “truth is in the depths,” says Democritus (DK B117).

How Democritus or Empedocles have access to such truth sufficient to articulate their theories is a question that they must answer. Democritus himself seems to realize this danger when he has sense experience rebut reasoning about the inaccessible depths of truth in this way: “Wretched mind, do you take your evidence from us and then throw us down? Throwing us down is a fall for you!” (DK B125). We can learn a twofold general lesson from this. First, responses to the Parmenidean-Heraclitean problems of change cannot resort to a self-undermining reductionist account of change (such as atomism). Second, natural philosophy must find a resolution to the problem that harmonizes our sense experience and our reasoning about things without setting them in a dualistic opposition.

The resolution to the Parmenidean-Heraclitean problem of change is a coordinated set of distinctions. The Parmenidean premises ignore a key distinction: that “being” and “non-being” are not of one type only. “Being” is said in more than one way; it is an analogous term. Something could exist presently (right now, “in act”), or something could exist in waiting (latently, “in potency”). Likewise, by “non-being” one could intend strict nothingness or one could mean a qualified sort of non-being, such as “There is no top button on that shirt.” This sort of nothingness is a privation but not utter non-being. Parmenides’s argument is not sound because it ignores these distinctions. Nothing comes from strict nothingness, but something can begin to exist where a lack of it was present before. If something actually exists, then of course it cannot come to exist again; however, the capacity for something to exist is itself a sort of being and can be realized in full presence or actuality. The Heraclitean premises, likewise, ignore the distinction between being in act and being in potency, as well as the distinction between remaining the same unqualifiedly versus remaining the same in some respect. The boy who grows up does not remain the same in height, for instance, but does remain the same person.

Every Change Needs Potency and Act

These distinctions, available to us along the order of determination in natural philosophy and necessary to resolve the twofold problem of change, lead us to insights about the principles of change: form, matter, and privation (see St. Thomas, On the Principles of Nature, chs. 1–2). Form is the principle of being in act; matter is the principle of being in potency; while privation is a principle of qualified non-being. Although such terms can become technical jargon, we avoid this by recalling our common experience and description of changing things. In order to change, physical things must be capable of achieving some feature or character which they currently lack. Realizing, in general, that things change because of potency, act, and privation is a necessary first step in the investigation of the natural order.

Changes of the features or characteristics of a physical substance are accidental changes (e.g., the boy growing in height). The form or act that is the principle of this change is an accidental form (a certain stature or quantity), while the matter or potency involved is the boy himself as subject of the change (the ability to grow because of the capacities present in his immature body). Some changes are more extreme, as when a physical substance begins or ceases to exist. The boy himself, of course, has not always existed. Such changes are called substantial changes. The form that is the principle of such a change is a substantial form and the potency involved is likewise the matter of the substance, called prime or primary matter. (We will return to this idea below.) The lack of greater height is a privation of an accidental kind, while the non-existence of the boy before his conception is a privation of a substantial kind.

The principles of form and matter not only give a general account of the act and potency involved in change, but they also account for the existence and ability of the substance in question once it comes into being. That is, form and matter are principles not only of becoming but also of being.{14} By contrast, since privation is a lack of being, it does not contribute something real to either becoming or being. Nonetheless, privation is necessary for change. Parmenides was right to say that a being cannot become what it already is. In order to reflect these different contributions of the principles of change to what comes to be, form and matter are called necessary and essential (per se) principles of change while privation is called a necessary but accidental (per accidens) principle of change.

Yet the general account of change cannot rest with these two or three principles. As St. Thomas argues in On the Principles of Nature (ch. 3), form, matter, and privation “are not sufficient for generation. What is in potency cannot reduce itself to act: for example, the bronze which is in potency to being a statue cannot cause itself to be a statue.” A sculptor is needed to bring the bronze statue from potency to act. The sculptor also needs a reason, a goal, and an end sought in sculpting. Thus, other causes are needed—bringing our total to Aristotle’s four causes—which we discuss below, in Section III.

The Importance of Principles

Before turning to nature’s causes, we should reflect on the importance of the insights which resolve the aporia about change. Through its resolution, we embark upon inquiry about nature properly speaking. This is because the intellectual life is one of reasoned insight into what and why things are. However, the aporia constructed by Parmenides and Heraclitus stymies rational inquiry by cutting it off from our sensible experience of the world. Parmenides is clear about this: only opinion can be had of the world of change, not true knowledge. Through Cratylus, Heraclitus’s argument moved Plato to hold a similar view. By showing how change is intelligible, at least in general terms, the Aristotelian solution avoids this result and opens up the possibility of deeper rational inquiry about the cosmos.

Yet it is only a principle, a beginning. It is clear to us from the beginning that things change, yet to explain this change in such general terms as potency and act cannot be fully satisfying to the student of nature who wonders why the sun shines or why an oak tree grows. Thus, we must conceptually strengthen and extend our grasp of form, matter, and privation to more accurately and adequately understand the natures of changing things.

Consider a first example of how such further inquiry can be provoked: the case of a particular ship, originally recorded by Plutarch in his “Life of Theseus.”

The ship on which Theseus sailed with the youths and returned in safety, the thirty-oared galley, was preserved by the Athenians . . . . They took away the old timbers from time to time, and put new and sound ones in their places, so that the vessel became a standing illustration for the philosophers in the mooted question of growth, some declaring that it remained the same, others that it was not the same vessel. (Plutarch’s Lives, 49)

How might the philosophy of nature, using the principles of change, approach such a problem? Apart from Theseus’ mythic ship, does this problem not return in quotidian guises, when it comes to growth, cell turnover in our bodies, or even organ transplants? The English philosopher John Locke, for instance, raises this line of inquiry when asking what makes a person the same person over time.{15} He finds the unity of a person in the continuity of consciousness: I am aware and recall my own thoughts and experiences up to today and so must be the same person. The Aristotelian answer would return to the unity provided by the substantial form of a physical thing; form as the act of this given matter is a principle of identity and unity. The principles of change thus open upon a field of questions about material composition and continuity of existence still present in philosophy, chemistry, medicine, and psychology.

Consider a second example: the case of Zeno’s dichotomy paradox, recorded by Aristotle in his Physics.

There is no motion because that which is moving must reach the midpoint before the end . . . . It is always necessary to traverse half the distance, but these are infinite, and it is impossible to get through things that are infinite. (Curd ed., 68)

The paradoxical argument that you cannot leave your own room sets up a series of divisions (1/2, 3/4, 7/8, etc.), a series defined by the rule (2n – 1)/2n (n starting at 1). The series never ends, since there is always another step to take (another n-th iteration of the rule). How might the principles of change address this argument? Is it the case that a moving object is actually present at a place when in motion through 1/2 the distance? Or is the thing only potentially present there, since it is in motion? Does actually occupying a place mean that the moving thing must be at rest? Exploring these and other questions about the quantitative structure of motion leads one into territory somewhat familiar to calculus and modern physics. Also consider that the intelligibility of the series is provided by the forms of the quantities involved. The form of quantity is a principle of intelligibility, today more frequently put in terms of units of measure, laws of motion, or the “information” in a system.

By considering the principles of potency and act, or matter and form, in more specific contexts, the inquiring philosopher of nature is led to make distinctions in order to gain more adequate insight into mobile being. What is more, these principles of change are extended in analogous ways in other parts of philosophy, in the various natural sciences, and are also present in Catholic theology. For instance, Catholic theology speaks of the form and matter of the sacraments. St. Thomas returns to the analysis of change and its principles when considering transubstantiation (see Summa Theologiae, III, q. 75). The solution to the problem of change is a seed which can grow and bear great fruit. The principles of change alone, however, are not enough. We need to investigate other sorts of causes.

III. Nature and Causality

A mobile thing is, first and foremost, a natural substance. This means that progress in natural philosophical inquiry cannot be attained without an understanding of nature and its causes. Insight into nature and what exists by nature or what happens by nature is also helpful to more fully understand contrasting cases, such as what exists only by human custom or technical intervention. A grasp of what nature is also helps us to better appreciate what is supernatural.

The Definition of Nature

To define in general the nature of a changing, physical substance, we need a few distinctions available to common experience. First, what exists because of human intervention is different than what exists by nature. A watch is not a natural object, while a salmon is. Second, what happens to an object due to principles intrinsic to it is different from what happens to an object from extrinsic sources. That the salmon swims upstream to mate is natural to it in this sense, while the salmon flying through the air because a grizzly bear has swatted it out of the water is not. Nor does the watch have an intrinsic source of change in this sense as a watch; the metal or quartz “tell time” because of the watchmaker’s art, while the metal rusts or the quartz accumulates electric charge intrinsically. Third, how a physical thing moves or acts precisely or primarily because of its nature is distinct from how it moves because of what it has, secondarily, in common with other things. Thus, a salmon migrating at a specific time to a specific spawning ground is what it does primarily, while falling to the ground (due to that grizzly bear), belongs to it in common with all heavy objects. With these distinctions in mind, we can define nature as follows:

Nature is a principle and cause of motion and rest in that in which it is primarily and per se, and not per accidens. (See Aristotle, Physics, II.1)

The salmon’s migration is a motion that arises from principles within it and primarily so. Aristotle adds “per se, and not per accidens” to exclude cases where the intrinsic principle of motion or rest merely happens to belong to this particular individual. It would not occur in the natural order always or for the most part. Aristotle’s example is a self-healing doctor, who treats himself precisely because of an intrinsic principle (his medical knowledge), but one which is per accidens to human nature, not present in every case due to what a human being is. Were the medical art natural, then one might expect to gain medical knowledge automatically in reaction to an injury; one might consider instinct as a contrasting case. That is, instinctive abilities do belong by nature, unlike abilities acquired through experience.{16}

As with the principles of change, defining “nature” in this way is very general. Further investigation is required to determine what, precisely, the nature of this or that thing is. One might turn this into an objection against the definition. If we cannot identify what is peculiar or proper to the behavior of a natural object in every case, then how can one have confidence in the definition? Furthermore, if our experience of natural objects is limited by place and time, surely we might misidentify what or how a thing moves and changes, leading us to theorize its nature incorrectly.

Such difficulties, while they have a point, ultimately miss the mark. To identify that physical things have natures is not the same as to identify precisely what those natures are. It is easier to see, at a general level, what belongs to an artificial thing or what happens due to interference in contrast to what is natural. Nonetheless, it is correct to think that knowing the general definition of nature is not the goal of the philosophy of nature but is rather one of its principles. Using the contrasts and distinctions available through common experience, we can see what natures are in general and then use this as a principle of inquiry in seeking a precise account of what things are. For this, natural histories are essential.{17} These inquiries into the details are open to error, but they are also open to correction and improvement.

Inquiry and Explanation through Four Causes

Since nature is a principle and cause of motion and rest, it is logically connected to the principles of change. It is also connected to the definition of motion (taken up below). The nature of a thing is both its form and matter, but primarily its form. To these we must add agent causality and final causality.{18} Let us consider these four in turn.

Form and matter together constitute the nature of a physical substance. Matter is “that from which something comes to be when it is in it,” like the bronze of a bronze statue; form is “the exemplar—the statement of the essence—and its genera . . . and the parts in the definition,” like the 2:1 ratio defining an octave (Aristotle, Physics, II.3). For instance, the salmon is composed of certain organic materials; it is made from flesh and bone; it is, like all known instances, a carbon-based life form; it is, like other substances with mass, composed of elements from the periodic table. However, this material causality only goes partway to making the salmon what it is. Just as we would not say that the “nature” of the watch is simply to be something composed of rubber, plastic, metal, and quartz parts, so too the nature of the salmon is not just its matter alone. The form of the watch is required to make it what it is so that it can work as it does and fulfill its function. So too, the salmon’s form is responsible for what it is and, by studying the salmon’s behavior and features, we are really getting to know its form. More wood might be available for making more bowling pins, but the quantity and quality of the matter are not sufficient to make a given number of any specific kind of thing.{19}

What exactly is form? Is form the structure or shape of the material parts in the whole substance? A salmon’s anatomy might be its form. Is form the ratio or quantitative balance among the material parts at a given time or over a period of time? A salmon’s chemical composition would be its form, or its biological functioning. Perhaps form is a set of capacities or powers which the salmon has during its existence, either in part or for all of its lifespan considered at once. These are all decent answers, and many of them and their variants have been proposed.{20} However, shape, structure, harmony, dynamic balance, and sets of powers are all accidents of a physical substance. They belong to or happen to the salmon by nature and thus cannot be the natural form of the salmon in question. They are effects, not the cause. The name “form” is used analogously in the case of substantial form, transferred from more knowable instances (such as shape or structure), to name the very “shape” and “structure” of the substance’s actuality precisely as a substance, which substantial actuality is known through its array of accidents and properties. The order of being is not the same as the order in our knowledge, and form is a cause in both orders—both of being and intelligibility.

Indeed, form is a crucial principle since, strictly speaking, we only know something insofar as it is in act. For instance, that someone has the ability to play the piano is known by his act of playing. The examples of matter in the salmon given above were all types of matter, matter with a certain form (such as its flesh or bones or carbon). That is, these material causes are called secondary matter, or matter existing under some form. The matter of a substance as such, primary or prime matter, is pure potency, lacking any form or kind.{21} How does one arrive at this conclusion?

Matter’s potentiality is knowable through form as its act: “The underlying nature is an object of scientific knowledge by analogy” (Aristotle, Physics, I.7). This is also true of secondary matter; for example, I learn about the accidental capacities of carbon or a salmon’s flesh through the forms it can or cannot take through a series of changes while itself remaining the same subject. As for substantial changes, what change reveals about substances is that, from prior materials, one substance comes to be. In the case of a dog, for instance, the potentiality of the canine gametes to bring about a single dog is proximately rooted in what they are (secondary matter), but the potency to be such materials is more remotely caused by prime matter and the forms to which it is in potency. Now, pure potency is a necessary feature of prime matter which we infer due to its immediate relationship with substantial form as a co-cause. Were prime matter not pure potency, then it would have a type or species, and thus a form. Were that form a substantial form, then additional forms would be accidental forms, contrary to the expectation that a substance such as a human being has its own substantial form. Were the form of prime matter an accidental form, then there would necessarily be some prior subject or substance with its own form. This would fail to explain the unity and reality of the whole as such: the substantial whole is not one composed of a complex of actually existing substances, nor is it a layered emergence of multiple substantial forms but a single form-matter composite.

That changeable substances are composed of substantial form and primary matter is a key thesis of the philosophy of nature. Mobile beings are hylomorphic composites.{22} The truth of this thesis is grasped both in the solution to the problem of change as well as in contrast to other options such as dualism or materialism (e.g., atomism). A dualistic approach would account for the act and potency required to solve the problem of change by treating these as distinct things or substances and not principles of things. A materialist approach might account for act and potency by treating them as distinct aspects of a material thing but not really distinct principles. It is not a brief argument to settle this dispute.{23} However, the realization that only hylomorphic composition preserves the unity of substances in our experience is a crucial insight. It leads to other theses which the philosophy of nature attains in its own way, but cannot fully explain, such as the unicity of substantial form.{24}

In addition to form and matter, the causal account of mobile beings would be incomplete without a discussion of agent (or efficient) causes as well as final causes. That movers and makers are required for things to change is evident in our experience: “that from which there is a beginning of motion or rest is a cause, such as the man who gave advice, the father as cause of the child, and generally what makes of what is made and what causes change of what is changed” (Aristotle, Physics, II.3). Induction supports the necessity of agent causes in both artificial and natural cases. The watch cannot self-assemble. The salmon or the dog or the boy all have parents. Water does not spontaneously boil without a source of heat. Inquiry into agent causes leads us to recognize distinctions between action, passion, and interaction. Agent causality joins physical substances through such relationships, forming environments, habitats, ecosystems, solar systems, and, eventually, a cosmos. Studying the details of agent causality leads to other distinctions among material things (e.g., some are breakable, soluble, ductile, deformable, etc.). Agent causality is not just a real type of causality, but the natural philosopher uses it as a basis for asking questions about natural things: How does it do that? A naturalist such as J. Henri Fabre investigating the instinct of insects is investigating how grasshoppers, wasps, and burying beetles behave as agent causes in their natural environments.

Since “everything which acts, acts only by intending something, there must be some fourth [cause]: namely, that which is intended by the agent; and this is called the end” (St. Thomas, On the Principles of Nature, ch. 3, translation modified). This reality of final causality, teleology, is clearest in our own experience of ordering our own actions or material things to achieve desired results. Yet actions for an end are not limited to the realm of human action. An agent intends something when it has a natural inclination to something. Thus, the salmon intends to reach its mating grounds. The tree intends to reach a certain height as it grows and matures. The final cause defines this intention as to its end. An end in this sense is also distinct from a mere stopping point or any result whatsoever. The natural end of a natural motion is a good of the mobile being in question (e.g., it is good for the boy to reach a mature height). Where something stops is not as such its good (e.g., if a boy’s growth is stunted or interrupted by injury). That such natural intentions are possible without knowledge allows us to distinguish human purposes from natural ends.{25} Our purposes and choices are shaped by our knowledge of the things we want. However, the intention of an end can be present apart from knowledge even in our case. Consider the muscle memory of an advanced guitarist who, unlike a beginner, plays with ease, without the need to pause and deliberate about each note.

By making use of all four causes as principles of his investigation, the natural philosopher can proceed along the order of determination, seeking more specific knowledge of things and hoping to thereby make progress along the order of demonstration and provide explanations for various effects. The philosophy of nature alone—in distinction from metaphysics or mathematics—uses all four causes precisely as causes.{26} Mathematics explains its subject using only abstract aspects of formal causality (quantity and properties consequent to quantity, such as equality). Even metaphysics uses matter only indirectly, since not every being is a material being.

Chance and Necessity Need Final Causality

What is determined by its natural causes is given definite constitution and direction in its activity. Such a thing is not determined or deterministic in every way, however. For “we observe that some things come to pass in the same way always, but others for the most part” (Aristotle, Physics, II.4). Causes can be impeded or can fail. What happens neither always nor for the most part but for the lesser part includes the type of event that is indeterminate or random: what happens by chance.

Let us first consider the cases of what happens always or for the most part. Such motions or behaviors are due to the natures of things, for by nature a physical substance is apt for and ordered to a certain range of outcomes. Nature is determined “to one,”{27} that is, to one set of effects typical of a given kind. For example, given certain pressures and temperatures, water may be a solid, a liquid, or a gas. This characteristic pattern of phases for water is unique to water; its nature is determined to that one pattern as opposed to another (e.g., that of oxygen, which has a different phase profile, given certain pressures and temperatures). Additionally, consider how a rock behaves if you hold it up and then let it go, versus how that rock would behave if an astronaut did the same thing in orbit. Such dependable results characterize what could even be called law-like behavior (we will discuss the “laws of nature” in Section VI.

Now let us consider what happens for the lesser part. The phrases “what happens always;” or “for the most part;” or “for the lesser part;” can lead to the mistake of framing this question entirely in terms of the sheer statistical frequency of occurrences. However, these phrases are easier to understand when used as an interpretive aid in our experience of the course of nature when characterizing the causes or dispositions of things. If you let the stone go, it will fall (always or for the most part) unless something stops it (which occasionally happens). What happens “for the lesser part” in this sense is apart from or besides the intention of what happens due to the nature of a thing. Thus, the chance event is a type of accidental being. What nature could have accomplished directly arises fortuitously, by happenstance. “Chance” names natural agent causes acting and accomplishing things accidentally.{28} For instance, you visit the market to buy apples and, by chance, run into a friend (versus having intentionally arranged a meeting). Or, a stone rolls down a hillside and, by chance, crushes a small lizard (a predator could have intentionally crushed the lizard). Chance events fill in the cracks, as it were, of all that arises by nature; they complete the universe in this way (see St. Thomas, Summa contra Gentiles, III.74).

This means that every sort of motion, whether arising by nature or due to chance, is causally dependent upon final causality, since every natural agent acts for an end, whether acting per se or per accidens. The famous dictum that “nature acts for an end” is the principle of others, especially “nature does nothing in vain.” Indeed, the final cause is the cause of causes, as Aquinas explains:

The end is not the cause of that which is the efficient cause, but it is the cause of the efficient cause being an efficient cause. For example, health does not cause the doctor to be a doctor—I am speaking of the health which comes about by the doctor’s activity—but it causes the doctor to be an efficient cause. Therefore, the end is the cause of the causality of the efficient cause, because it causes the efficient cause to be an efficient cause. Likewise, the end causes the matter to be the matter and the form to be the form, since matter receives the form only for the sake of the end and the form perfects the matter only through the end. Therefore, we say that the end is the cause of causes, because it is the cause of the causality in all causes. (On the Principles of Nature, ch. 4)

In other words, without health as a form realizable through medical activity, the doctor as such could not act medically at all. The form as realizable finalizes the activity as its good. Due to health as an end, both the matter (a healable human patient) and the form (health actually possessed) are explicable and able to be causes. The matter is a healable human patient: health as an end makes this potency to be such a potency insofar as act is the end of potency.{29} The form is health itself, the perfection of the healable patient, and thus the relevant form the doctor instills, as opposed to some other form.

But should we trust that teleology is a real principle and cause in nature? A host of objections might be raised.{30} For instance, one might say that the agent cause explains the entirety of the effect, so final causes are superfluous. However, something can have more than one cause, and, as just noted, agents are agents due to the ends for which they act. There are more difficulties one could raise, and we will consider some of these after a positive statement of the case.

First, it would help to clarify exactly what “nature acts for an end” means by stating what it does not mean. It is not a claim that natural objects are designed, nor is it a claim that we read off providential purposes in things. It is also not merely a claim that things have natural inclinations or “intentions” in the sense discussed above. Rather, it is the claim that the natural inclinations of things are such because of an end-goal, which end is some good of the natural thing in question. The end or final cause is the cause of causes; it completes the intelligible structure of the agent causality of the natural thing. Without attending to such a cause, the natural philosopher would have an incomplete account of nature.

It would be helpful to review Aristotle’s arguments in defense of the claim that nature acts for an end, found in Physics, II.8.

In each of these arguments, some end as a good is indicated: (1) Nature frequently achieves what is good, and chance cannot explain this; (2) The order in a natural process arrives at a good such as maturity or a healed wound; (3) The agent’s goal of this form in this matter, like art, realizes a completed thing, and completion is good; (4) Parts and wholes of living things aid the goods of survival and flourishing; (5) The form of things is a perfection (complete act) of matter, and this existence is a good. The “good” we are speaking about here is a word which we might first use in reference to the fulfillment of our own appetites, but that itself is a specific case of the general claim: the completion of a potency by the act for which it is apt. In this instance, we cannot confuse the source of our own familiarity with the name “good” or “end” with the very meaning of “the good.” “Good” is a transferred or analogous term; we apply the name to other ends of natural processes not because they satisfy something like human desire in those things, but because those goods, like the goods in human experience, are completions of a potentiality by its proper act.

We noted above that further difficulties could be raised against the claim that nature acts for an end. We consider some of them in Table 2.

We should note four things before concluding this discussion of finality in nature. First, final causality is inextricably tied to the Aristotelian notion of nature; in other words, that nature acts for an end is true per se given what nature is in the Aristotelian sense. Speaking of theories such as those of Empedocles (who explained the origin of living things through chance), Aristotle argues that

the person who asserts this entirely does away with “nature” and what exists “by nature.” For those things are natural that arrive at some completion by a continuous motion originated from an internal principle: the same completion is not reached from every principle; nor any chance completion, but always the tendency in each is towards the same end, if there is no impediment. (Physics, II.8)

That is, cases such as chance or violence are definable and exist precisely in contrast to what exists by nature. Violence is what is against nature; a deformity is what is beside the intention of nature; and a chance event is brought about by natural agents accidentally. Even an accidental cause such as chance can be subsumed by a natural purpose, just as art makes use of chance. The hunter does not care which shotgun pellet brings down the duck, and the oak tree produces far more acorns or the dandelion far more seed pods than can grow into mature plants, since many will be impeded.

Second, it is sometimes objected that Aristotle’s comparison of artistic or technical procedures to nature vitiates his point. However, making use of the principle that art imitates and perfects nature (see argument (3) in Table 1) is an argument from what is better known to us. That principle is true because nature acts for an end, but we are able to realize that nature is such because of our familiarity with art. What is more, human art and technology could not imitate or complete nature unless nature itself were open to such ends, containing our ends in advance, as it were. For human reason is also a principle due to human nature. It bears emphasizing that this ordering of nature as principle and cause to an end is the basis for discovering, as a further insight, the need for intelligence to so order natures.

Hence, since nature has the determinate means by which it acts, it does not deliberate. For nature seems to differ from art only because nature is an intrinsic principle and art is an extrinsic principle. For if the art of ship building were intrinsic to wood, a ship would have been made by nature in the same way as it is made by art . . . . Hence, it is clear that nature is nothing but a certain kind of art—namely, the divine art—impressed upon things, by which these things are moved to a determinate end. It is as if the ship builder were able to give to timbers that by which they would move themselves to take the form of a ship. (St. Thomas, Commentary on Aristotle’s Physics, bk. II, lect. 14, n. 268)

Nonetheless, accepting such a point about the divine origin of natural ends is not necessary in order to see the truth of the principle that nature acts for an end. The defense of teleology in Physics, II.8 can lead to but is not the equivalent to or the replacement of considerations of the sort St. Thomas proposes in his famous Fifth Way of demonstrating the existence of God.

Third, consider again the claim that what happens by material or absolute necessity is not incompatible with final causality. The objection against final causality in this case was that matter itself gives rise to results determinately or deterministically. However, in order to achieve a result with certainty, it is precisely an instance of “order to an end” to use means which will not fail. Matter of its nature is open to many possible results, and so its use for one end will bring about the possibility of undesirable side-effects, such as rust on a metal tool, or the necessity of death for an organism. But such a range of alternative possibilities does not negate the reality of finality. What matter necessitates in this order of generation is absolute or material necessity. The necessity of certain materials in order to achieve an end is hypotheticalor conditional necessity. In this way, the determinism of events, often touted based upon hasty generalizations from Newtonian or classical mechanics, misses half of the picture of what it means for causes to necessitate.{31}

Finally, while the natural ends or goods of the inorganic realm are far more limited, they nonetheless exist. As St. Albert the Great states about stones, “We need not look for a final cause, since in physical things the form is the final cause.”{32} Wallace also notes:

Perhaps one should differentiate here between (i) processes that are good for a particular nature, say, to conserve it in being, the way in which salt crystallizes and so preserves its identity, and (ii) those that are good for nature as a whole, the universe being made up of many different kinds. Elements are good in themselves, but compounds may better or more readily serve the needs of the organic world; plants and vegetables represent a higher stage of being than complex molecules, but less than that attained by the animals that eat them and incorporate them into their substance. (The Modeling of Nature, 17)

Thus, whether in their own forms or in their aptitude to serve in the compositions of other things, the good is still present in the non-living world.

IV. Investigating a Cosmos Full of Motion

At this point, a brief summary is in order. The philosophy of nature begins the study of mobile being by addressing in general the necessary conditions for change to occur. This leads to initial insights into the principles of change (form, matter, and privation) and to the further realization that change arises by nature due to four natural causes. These elements of natural philosophy would not be complete unless motion itself were also understood: “We must therefore see that we understand the meaning of motion, for if it were unknown, the meaning of nature would also be unknown” (Aristotle, Physics, III.1). That is, we cannot understand nature as a principle and cause of motion and rest without a definition of motion. We will consider how the philosophy of nature investigates motion in two stages. The first concerns motion itself as well as its species and concomitant properties (this section). The second asks about the ultimate origins of motion (Section V).

Think of boiling a pot of water over a campfire; a natural process would be a body of water being warmed by the sun or heated by a lava flow. The water itself is capable of being heated, and after it has been heated it is capable of retaining that heat which it actually possesses (at least for a time). The water is the moved thing, undergoing or suffering a passion of being heated; the source of heat is the mover, and its action is the principle of the motion.{33} While the water is heating up, what sort of reality is this? It seems to be some reality in between the potency of being heatable and the actuality of possessing heat:

Consider, therefore, that something is in act only, something is in potency only, and something else is midway between potency and act. What is in potency only is not yet in motion; what is already in perfect act is no longer in motion, but has ceased to be in motion. Consequently, what is in motion is midway between pure potency and act, that is, it is partly in potency and partly in act—as is evident in alteration. (St. Thomas, Commentary on Aristotle’s Physics, bk. III, lect. 2, n. 285; translation modified for clarity)

Aristotle defines motion as exactly such a “halfway” reality: motion is the actuality of what exists in potency as such. On a first reading, this definition is not exactly transparent; indeed, it is rather impenetrable. Nonetheless, Aquinas characterizes the definition as follows: “It is entirely impossible to define motion in terms of what is prior and better known otherwise than the Philosopher here does” (ibid.). So, let us carefully approach a more complete understanding of the definition.

Consider how other philosophers have approached an account of motion.{34} Some have proposed that motion is not a reality (Parmenides, Zeno), while others think that motion is real but not really distinct from rest. Bertrand Russell defends the “at-at” view of motion, in which a moving object, like frames in a film reel, occupies one place at each moment of time. Others think that motion is real and really distinct from rest, but that there is no need to define it because it is intuitively clear (e.g., Descartes’s geometric conception of motion). Some think motion should be defined, but do so circularly, using terms which themselves imply motion (e.g., Newton in his Principia). Yet others consider that motion is definable but only through a certain contradiction (e.g., Hegel, Marx). The Aristotelian view alone holds that motion is a reality distinct from rest and definable in a non-circular way through non-contradictory notions.

How should we understand Aristotle’s definition? First, it defines motion as a reality: it is an actuality.{35} Now, to any potency there is a corresponding act.[36] Motion, like other acts, must be the act of some potency. But which one? The act of the healable as healable is becoming healthy; the act of what is buildable as buildable is building; the act of the heatable as heatable is heating up. Or, so goes the typical Aristotelian induction, hoping to clarify the matter.

To further understand the definition, we can proceed by a process of elimination.{37} Consider that a motion has a beginning and an end, a whence and a whither, a terminus a quo (term from which) and a terminus ad quem (term to which). Where is the “potency as such”?

a. Is motion the act of the potency of the subject itself? No. For if it were, then to be that mobile would be to be in motion. As Aristotle states, “If they were identical without qualification, meaning in definition, the act of bronze as bronze would have been motion” (Physics, III.1).

b. Is motion the act of the potency to exist in the terminus a quo? No. For if it were, then motion would be the same as rest. This corresponds to the pot of cold water prior to exposure to the fire.

c. Is motion the act of the potency to exist in the terminus ad quem? No. For to exist in the terminus ad quem is to have completed a motion. The motion is over at that point. This corresponds to the pot of water reaching a boil. The process of heating is over.

d. Is motion perhaps the act of the potency to exist at some midpoint between the terminus a quo and the terminus ad quem? No. For if motion were the act of this potency, then motion would be a rest or a stopping point midway through the motion. This corresponds to the water ceasing to be heated and now being only lukewarm.

Note that a potency does not cease to be when its act or form is present. When I am standing, I also have the potency to stand. (It would be odd to say that I am unable to stand when standing!) I have the potency to stand while I am sitting, of course, but this potency is composed with the privation of standing. The act of a potency without a privation is what obtains in the cases above. We need another option:

e. Motion is the act of the potency as such, or, inasmuch as the mobile is still in potency. This “as such” is meant to emphasize that the potency in the definition of motion must be understood with privation, namely, the potency of the terminus ad quem precisely as existing with privation, as not yet achieved.{38} The act of the water heating is the act of potentially boiling water, understood with privation. When this privation is no longer present, the motion has reached the terminus ad quem.

This accords with our experience of motion. For instance, as I walk through a doorway, my act of being “in” the doorway is different than the act of the person simply standing in the doorway. My act of being in the doorway is coincident with the privation of the place to which I am walking precisely as such; the other person’s act of being in the doorway also has a privation of being in other places, but that privation is not presently ordered to any one of those potential places as such through an act of motion.

Just as is the case with our grasp of the principles of change or the account of nature and the four causes, budding philosophers of nature cannot stop at the general definition of motion. The definition reveals something true about the world, but it is also a principle of further inquiry. What is involved in the act of motion? Is it countable or measurable? Does the act of motion always require a mover, or could it arise spontaneously? Is the potency for a motion different in every case? Does the potency for a motion run out eventually, or might it be indefinite?

To understand motion more fully, one must investigate these questions. One should also ask about certain properties and realities concomitant to motion: infinity, place, void, and time, as well as the relationship between mover and moved. These align with our common experience and reflection. For example, Zeno’s dichotomy argument illustrates motion’s connection with infinity: experience tells us that all motions occur in time. Clarifying the reality of the place of bodies (like the habitats of living things) or understanding the nature of time as the measure of motion depends upon taking the reality of motion seriously. For instance, the reality of motion counts against philosophical interpretations of scientific theories which deny the reality of time.{39} Mathematical theories of motion, much like line drawings in a comic sketch, leave out much of the physical reality of things.{40} Sorting out the definition of place and contrasting it with the nonexistent, void-like space; puzzling over the definition of time; or considering the necessary quantitative conditions of mobile beings—especially for local motion—lead the philosophy of nature into the territory of the philosophy of physics.{41}

Qualitative changes or alterations, as well as the substantial changes to which they are preludes, must also be investigated. It is true that Aristotle’s beginnings here were plagued by a certain selection effect, namely, defining elements in terms of tangible qualities like hot and cold or wet and dry. However, it is a question worth considering whether or not all types of change are simply local motion under different appearances. Similarly, it is a question worth considering whether or not substantial changes are simply the structural rearrangement of fundamental particles or atoms. Against such varieties of reductionism, the Aristotelian philosophy of nature proposes, for instance, that the parts of a composite substance are really present in it but not actually so; rather, they are present in virtute, or in power. Similarly, certain powers and capacities when activated seem to underlie qualitative changes, from electric forces to physiological changes. Such questions lead the philosophy of nature into the territory of the philosophy of chemistry (and a bit of biology).{42}

It belongs to a specific part of the philosophy of nature to investigate living things, in particular the human being (at least up to a point). The Aristotelian-Thomistic philosophy of nature argues that all living things have souls, and by “soul” is meant a living thing’s substantial form. More precisely, the soul is the first actuality of a material body potentially alive in virtue of its organization. Like other places in the philosophy of nature, we begin with something clearer to us at first, namely the experience of being alive and the various activities which we associate with life, such as self-motion, growth, reproduction, sensation, and knowledge. (As an aside, one must pause here and reflect that such a beginning of the philosophy of motions associated with life cannot succeed with a scanty or topical natural history of living things.) There must be some principle of these effects in substances we can identify as “living” in one or more of these senses. Such a principle cannot be bodily, as Aquinas argues:

It is manifest that not every principle of vital action is a soul, for then the eye would be a soul, as it is a principle of vision; and the same might be applied to the other instruments of the soul: but it is the first principle of life, which we call the soul. Now, though a body may be a principle of life, as the heart is a principle of life in an animal, yet nothing corporeal can be the first principle of life. For it is clear that to be a principle of life, or to be a living thing, does not belong to a body as such; since, if that were the case, every body would be a living thing, or a principle of life. Therefore a body is competent to be a living thing or even a principle of life, as such a body. Now that it is actually such a body, it owes to some principle which is called its act. Therefore the soul, which is the first principle of life, is not a body, but the act of a body; thus heat, which is the principle of calefaction, is not a body, but an act of a body. (Summa Theologiae, I, q. 75, a. 1)

This “act of a body” is the sort of act which makes it to be alive: it is a substantial form of a certain sort. In order to investigate the nature of this form, we must attend more closely to the varieties of activities of living things and the things to which they are directed or work upon or are affected by: their objects. We discover the nature of the soul and its powers through reflection upon the objects and activities of those powers. The natural philosopher’s inquiry into the soul goes on to distinguish sensation as a type of cognition from mere physical qualitative changes; it argues that intellectual knowledge cannot be explained by a physical power or organ.{43}

What is more, “we are ignorant of the soul to the degree that we are ignorant of the body of which it is the first act” (De Koninck, “Introduction to the Study of the Soul,” 55). The natural philosophy of living things and their changes cannot rest, then, in general claims about the soul and its powers, but must satisfy itself with the gory details of biology, anatomy, physiology, etc. Such topics open the philosophy of nature to a philosophy of biology.{44}

In these various ways, in both the orders of determination and demonstration, the philosophy of nature leads us towards the various natural sciences. The general study of motion and its principles and natural causes opens upon a natural philosophy of subjects found in physics, chemistry, biology, etc. However, before reflecting upon the implications of this “downward” path, we should consider the one that goes “upwards.”

V. Ultimates in Natural Philosophy

The philosophy of nature makes use of all four causes in its explanations of mobile beings in the cosmos. As just suggested, as this inquiry becomes more detailed and complex in the material order, it approaches the various branches of the natural sciences.

The dependence or passibility of some mobile beings in relation to others is clear in our experience, especially when it comes to living things and their habitats. Erosion, sedimentation, evaporation, condensation, and other inanimate processes, from earthquakes to the weather, are sustained through the agency of vast collections of substances, affecting the environment and living conditions of animate substances. Inorganic materials are required for the existence of the organic materials needed by living things. Gravity and heat (largely from the sun as an energy reservoir) underlie many of these processes involving fluid flow, pressure, or heat exchange. The sun, in turn, is presently conditioned in its motion and place by other astronomical systems and required other causes to bring it into being through the process of star formation. “The order of the universe,” Aquinas writes, “is made up of the order and connection of causes” (On Truth, q. 11, a. 1, c.), most conspicuously agent causes and their interrelationships along with various ends in sequences of subordination or hierarchy. 

While an Aristotle or a St. Thomas would complete the sequence of local causal order against the backdrop of a geocentric cosmos, their general argument in natural philosophy from the existence of motion through sequences of connected movers to a first mover remains available for us today, living in an expanding, Big Bang cosmos. Indeed, this proof of the existence of God from motion still has its defenders.{45} The success of this proof offers, on the part of natural philosophy, the means for reason to fulfill its duty to faith, providing the believer with one of faith’s preambles (the praeambula fidei), for “God, the beginning and end of all things, can be known with certainty from the consideration of created things, by the natural power of human reason” (Vatican I, Dei Filius, ch. 2).

The argument, in general, is as follows: Whatever is in motion must be put in motion by some other mover, distinct from itself. If that mover is itself in motion, it must be put in motion by some mover prior to it, causally speaking. That is, movers which must be in motion in order to be movers derive their power to be movers from some other mover. Such a sequence of causal dependence of moved movers cannot be indefinite. Therefore, there must be some first mover which is itself not in motion. This first, unmoved mover we call God.

Each step in this argument demands a more careful and extensive examination than can be given in an introductory text devoted to the philosophy of nature’s general scope.{46} However, three notes should be mentioned. First, while the exact status and meaning of the proof in the texts and the minds of the historical Aristotle or Thomas are points of contention amongst scholars, the philosophy of nature as a discipline intrinsically requires such a proof. Its investigation of motion and mobile being naturally leads it to ask about the origin of motion as such, as a universal effect. Its inquiry would be essentially incomplete without the proof of a First Mover from the existence of motion or some equivalent resolution to a universal explanation or cause proportionate to the universal effect of motion. (For instance, a naturalist might resolve the existence of motion to a brute fact, or, perhaps, to the law-governed character of all motion, the laws of motion taken to be ultimate. But some answer must be found.)

Second, even St. Thomas notes that the argument from motion reaches a limited conception of God that must be further clarified (see Summa contra Gentiles, I.13). As the great Thomistic commentator, Cardinal Cajetan, notes, in “the first way, on the part of motion, it is enough that it infers ‘therefore, there is a first immobile mover,’ not attending to whether that [mover] be the soul of a heaven or one of earth, for this is asked in the following question” of the Summa Theologiae, namely Question 3 (Cajetan, In ST I, q. 2, a. 3). Nonetheless, “first unmoved mover” suffices to name God as a cause, even if it is not a name sufficient in every way. For, in truth, God is the first unmoved mover, albeit further arguments are needed to clarify and distinguish His mode of being an unmoved and wholly immobile mover from other, more limited and merely relative first movers in the cosmos.

Finally, given the soundness of such a proof, it follows that natural philosophy discovers one of its limits. That is, it discovers the existence of a first mover which is immobile in every way, possesses the power to sustain motion in the cosmos without fail, has no bodily magnitude (since bodily limits would limit its power to sustain motion in the cosmos), and, indeed, is

wholly without magnitude, as though existing outside the genus of magnitude. And thus does the Philosopher [Aristotle], in his general consideration of natural things, terminate at the first principle of the whole of nature, who is the one above all things, the ever-blessed God. Amen. (St. Thomas, Commentary on Aristotle’s Physics, bk. VIII, lect. 23, n. 1172)

The philosophy of nature reaches a limit with the declaration that there exists some cause which is not a material thing; it is able to make such a claim on the terms of the motion proof, a natural philosophical articulation of the ultimate necessary conditions of motion as such. Since the First Mover is “outside the genus of magnitude”—that is, outside the genus of mobile beings—the philosopher of nature cannot study it beyond the recognition of its existence as a mover and the deduction of certain attributes by way of negation. The study of being which is not material, or the study of being insofar as it is immaterial, must belong to another part of philosophy (i.e., metaphysics).{47}

VI. The Philosophy of Nature and Wisdom

Aristotle teaches that “the philosophy of nature is a kind of wisdom, but it is not the first” (Metaphysics, IV.3, 1005b1–2). Indeed, the philosophy of nature is both a science and a qualified form of wisdom.{48} It treats of mobile being through its causes and is thus scientific in an Aristotelian sense in that it considers mobile being in light of ultimate causes, while directing the work of more specific lines of inquiry in the natural sciences, and is thus sapiential. In this section, we will consider these characteristics of the philosophy of nature in more detail.

As Part of Speculative Philosophy

The philosophy of nature is one of three parts of speculative science, according to the traditional Thomistic division of the sciences or parts of philosophy ordered to knowing the truth for its own sake and not for the sake of practical action or the making of artificial things.{49} This division is made based upon the intelligible character of the objects studied and the way in which we understand them. The things which become objects of speculative inquiry may or may not need matter in order to exist. With regard to the mind, we are able to contemplate things with certitude insofar as they cannot change due to materiality. Since things are intelligible insofar as they are in act, objects of the mind’s contemplation are differently understandable insofar as we think of them in different degrees of remove from potency or materiality. Thus, abstraction or separation from matter in thought or understanding is necessary. This yields the following possibilities:

There is no intelligible object corresponding to one of the options (there is no (D)), because it is not possible to coherently and truthfully think about a being that does not need matter in order to exist but needs it in order to be understood. Since this division’s principles take into account the nature of knowledge based upon both the thing known and the knower, it seems exhaustive.

The philosophy of nature resides in (A), considering those intelligible objects which it understands along with matter. Thus, natural philosophers must include in their definitions the “sensible matter” of things.{50} When I want to understand a mathematical object, I need not consider what material it is made of. By contrast, when I wish to understand salmon, I cannot leave off their material composition. I can safely ignore, however, whether or not a salmon is here or there and when exactly it exists; that is, I may abstract from its matter at the individual level of the hic et nunc (the here and now).

Three comments are in order. First, because these three modes of intelligibility characterize the terms of discourse in these sciences, they are, as it were, the lights in which their objects are knowable (three distinct formal objects). To speak about things as a mathematician is not commensurate with the way in which the natural philosopher speaks about things (for the natural philosopher is able to say more and thus comprehend mathematical intelligibility within his own mode). Furthermore, to speak about things as a natural philosopher is not commensurate with the way in which the metaphysician speaks about things (for the metaphysician is able to say more and thus comprehend natural philosophical as well as mathematical intelligibility within his own mode). 

Second, note that the division provides a reason why the philosophy of nature is necessary. Without it, a possible object of knowledge would be left out, since (B) mathematics does not study the material conditions of change, and (C) metaphysics could only consider material beings insofar as they are beings but not precisely as material. (To respond that there may exist, in its stead, applied mathematics or even applied metaphysics simply raises the question of why there is no inquiry that studies objects in (A) on their own.) Because of its necessary role in contemplating the being of the material universe, the philosophy of nature was once called “cosmology,” although that name is now infrequently used.

Third, note that the division separates the philosophy of nature from metaphysics and seems to join it more closely to the sciences of nature with which we are more familiar. Is this the correct way for metaphysics, natural philosophy, and the natural sciences to be related? Thomists who debate this question usually take one of three positions.{51}

(I) The philosophy of nature is a type of applied metaphysics and is thus either a part of (C) or some sort of middle ground between (A) and (C), with the natural sciences comprising (A).

(II) The philosophy of nature and natural science are two different species within the division (A) as a type of genus.

(III) The philosophy of nature is the science which studies the intelligible objects in (A), while the natural sciences are more specific continuations of this inquiry.

The principal difficulty with the first position (I) is that metaphysics is not prior to the philosophy of nature in the order of learning. As discussed above, we must begin with what is more knowable to us at first (all else being equal),{52} and this is not metaphysics. This is because we do not at first know of the existence of being which does not need matter in order to exist. As Aquinas argues, “our intellect is not proportionate to knowing by its natural light anything except through sensible things. And therefore it cannot come to purely intelligible things except through argumentation” (Sentences, bk. 1, d. 3, q. 1, a. 2, ad 2). Nonetheless, the philosophy of nature does provide our first encounters with the principles and concepts (such as potency and act) that metaphysics then tropes into a fuller domain: “Nor is there necessarily a vicious circle because metaphysics presupposes conclusions proved in the other sciences while it itself proves their principles. For the principles that another science (such as natural philosophy) takes from first philosophy do not prove the points which the first philosopher takes from the natural philosopher, but they are proved through other self-evident principles.” (St. Thomas, On Boethius’s De Trinitate, q. 5, a. 1, ad 9). Before the formal acquisition of a metaphysical habit of contemplation, the philosophy of nature overlaps materially with subjects it later shares with metaphysics.

Nevertheless, insofar as first philosophy can formally defend, order, and judge the principles of the other sciences, it could be called a philosophy of nature. Due to the different modes in which the intelligible objects of these parts of speculative philosophy are conceived and defined, such a use of “philosophy of nature” would be distinct from the sense of the term discussed in this book. Indeed, a yet broader use of “natural philosophy” is found among the medievals. Both St. Bonaventure and St. Thomas collectively call the generic group of physica, mathematica, and metaphysica “natural philosophy.” In this sense, it is defined as the study of objects which we find already given in the nature of things, in contrast to inquiries regarding human thought or conduct in logic, moral philosophy, or the technical arts.{53}

Position (II) could distinguish the philosophy of nature from the natural sciences as species within (A) in a variety of ways.{54} For instance, one could say that the natural sciences remain more at the level of the inductive study of sensible particulars and, insofar as they do offer causal explanations, these are had by way of mathematical models or hypotheses. In neither case does one rise to the level of the universal and causal consideration of things required of the philosophy of nature. The principal difficulty with this second position is that it proposes a difference in the methods or tools used to study material, mobile being but does not propose a difference which would mark out a new intelligible object (a new light in which things are known). For instance, the physicist who wishes to model the motion of the planets using Newtonian mechanics must still assess the accuracy of his considerations through the use of measurements keyed to physical units. In this way, he still defines his object with sensible matter. Even if his is a probable conclusion (i.e., one true only up to a certain point or within a certain range of accuracy), the physicist is working in the domain of the philosophy of nature.{55} The strength of this second position, however, is that it highlights the many ways in which the technical methods of the natural sciences specify the general approach of the philosophy of nature, especially in the emphasis it places on the “middle” or “mixed” sciences of the Greeks and medievals (e.g., astronomy or optics), involving both mathematical and natural aspects, which are the ancestors of contemporary mathematical and experimental sciences.

What has the third position to say for itself? Along the path from the generalities of the Physics to the other parts of the philosophy of nature, we should recall the interplay between the order of determination (conceptual specification and concretion) as well as the order of demonstration. Progress in natural philosophy is not attained by deducing more specific ideas and explanations from general ones (as if we were to argue “the soul is the first act of the body, therefore, a lion is such and such”). Rather, these general principles locate and focus our return to sense experience for further distinctions, phenomena, and explananda. To rest in formal, conceptual descriptions of the substances which compose the cosmos would be to be satisfied with a dialectical grasp of things, an understanding both tentative and subject to flux.

It is by an ever-deepening experience that the mind emerges little by little from this dialectical condition. In this respect, the treatises nowadays designated as properly constituting the philosophy of nature are at bottom only an introduction to knowledge of nature properly speaking. The philosophy of nature seeks to know what natural things are, not in a confused manner, but in their proper concretion. The unity of that end is not broken by the diversity of means employed. On the contrary, it is the same end that governs them, provided that they enable us better to know. (De Koninck, Writings, vol. 1, 449)

That is, as both their end and their general means fall within that of the philosophy of nature, the natural sciences are joined to it as specific sub-domains of the general intelligible object defined in (A). Indeed, the philosophy of nature ought to orient its student towards these specific sub-domains which define the various natural sciences. After all, those sciences either explicitly or implicitly assume the answers to the questions natural philosophy asks at a more general level and seek to improve upon such answers in terms of detailed, explanatory reach. There are, of course, differences in vocabulary between someone deep in the weeds of quantum theory and the discussion of material causality in an introductory philosophy of nature course; nonetheless, both are asking questions about the manner of existence of mutable, material things. Differences of approach, training, technical vocabulary, or a history of estrangement between philosophy and the sciences do not generate a difference in the object of speculation.

The philosophy of nature is scientific insofar as it attains to causal explanations at general or specific levels. This is the Aristotelian notion of a science: certain knowledge through causes (cognitio certa per causas). Why call it a form of wisdom? On the one hand, it is a kind of wisdom insofar as it befits wisdom to consider first causes in a universal way. Since metaphysics considers the first causes of things in a deeper and more universal way, it partakes more of the nature of wisdom. On the other hand, it also lays claim to the nature of wisdom insofar as the philosophy of nature at its general level directs inquiry in more specific levels, clarifying and helping to articulate their principles or relating them to other domains. Since metaphysics or first philosophy does this in a higher way, we must say with Aristotle that the philosophy of nature is a kind of wisdom, although not without qualification.

Sapiential Relationship with the Natural Sciences

In its sapiential office, the philosophy of nature is a philosophy of the sciences. We should consider this in a brief example.{56} What is a law of nature? While “natural law” or “law of nature” is properly used in the moral realm, its meaning can be extended to substances incapable of free choice. While such a usage as “laws of nature” would be metaphorical to the medieval mind, we seem to use it quite literally. So, what are laws of nature in the physical sense?{57}

Some argue that laws of nature are universals. For instance, all heavy objects fall down in a gravitational field near the earth’s surface. But critics point out that a universal as such does not seem to necessitate how objects behave. Nor is every universal description a physical law in the way we mean the phrase. Others argue that the laws of nature are regularities (i.e., what usually happens). Yet critics note that the use of the term “regularity” simply renames law-like behavior without defining it. It also does not take into account the possibility of impediments. Others suggest that laws are regularities described counterfactually (i.e., what would happen regularly in the absence of impediments). Still others think that the laws of nature describe the causal dispositions of things: all heavy objects are apt to fall due to gravity while certain causes are able to prevent this.

This last view draws us towards Aristotelian natures: “For natures just are abstract essences in concrete operation,” and the cosmos or “nature is the collection of all the natures of things. So, to say the laws of nature is to say that they are of the natures of things” (Oderberg, Real Essentialism, 144). If the laws of nature are true—if partial—descriptions of how objects behave due to their natures, it also makes sense why the laws of nature sometimes seem to be universals, or regularities, or to be limited by exceptions. That is, the natures of things are intelligible in a universal way; further, what happens by nature arises always or for the most part, or, with regularity; lastly, even what happens by nature allows exceptions (what happens for the lesser part). In this way, the core principle of the philosophy of nature helps to clarify the meaning and mode of causality of the laws of nature used in the natural sciences.

The cosmos is vast in the beauty of its intelligibility. We require a corresponding array of intellectual habits to contemplate it even in part. Among the topics and questions mentioned above, habits of logic, metaphysics, and mathematics, as well as the philosophy of nature must be brought to bear. The philosophy of nature benefits from these habits in various ways, drawing upon them alongside its own efforts to understand mobile being. We who would be philosophers of nature must be attentive to nature’s details and unsatisfied with generic descriptions: “We therefore must not recoil with childish aversion from the examination of the humbler animals” and plants and non-living substances which compose the cosmos (see Aristotle, Parts of Animals, I.5).

Relationship to Practical Philosophy

Besides its relationship to the natural sciences, the philosophy of nature also has a bearing upon practical philosophy, where it ministers to the discovery of various principles. In this way, the philosophy of nature is useful in a theoretic sense.

For example, the basis for Aristotelian virtue ethics lies in the definition of happiness as virtuous rational activity of the soul. This definition is arrived at through the use of what is often called the function argument: the good for each kind of thing is the proper work or activity (the function) of that thing, and the proper work of a human being is activity of the soul in accord with reason. This activity is better if it is excellent or virtuous. Behind such an analysis lies the principle that nature acts for an end; thus, the nature of a thing orders it to the motions and activities proper to it. In order to further explore this definition of happiness, we cannot make progress and “attain perfectly the science of morals unless we know the powers of the soul; this is why, in the Ethics, the Philosopher assigns certain virtues to the different powers of the soul” (St. Thomas, Commentary on Aristotle’s De Anima, bk. I, lect. 1, n. 7).

Political philosophy, too, benefits from such analyses. It relies upon the principle that human nature exists and is of such a character as to be apt for political considerations; for political philosophy “does not make men, but takes them from nature and uses them” (Aristotle, Politics, I.10). Medical ethics also relies upon a natural philosophical understanding of the human body. How else could it assume health as a normative paradigm, or certain biological functions as natural ends?{58} Lastly, while the contemporary natural sciences are more frequently associated with a fixation upon technical applications, the philosophy of nature can provide a broader, contemplative ambit for considerations of ecology and for understanding man’s proper place in the cosmos.{59}

VII. Conclusion

The task of the philosophy of nature is the causal knowledge of mobile being in general, leading its students towards the specific realms of inquiry that branch into the natural sciences. Its “fullness of contemplation” (St. Thomas, Commentary on John, n. 9) bears upon metaphysics as well as the other parts of philosophy. Nor is it unrelated to sacred theology. This relationship was anticipated in ancient pagan thought, contemplated in medieval Christendom, and still has relevance today.

As for the ancients, there is of course Aristotle’s judgment that, “by disclosing to intellectual perception the artistic spirit that designed them,” our studies of natural things, especially living things, “give immense pleasure to all who can trace links of causation, and are inclined to philosophy” (Parts of Animals, I.5). The pagan philosopher and Aristotelian commentator Simplicius, living in the age of Christian Rome, had some notion of the connection. Natural philosophy “kindles to the highest point our respect for divine transcendence by awakening [that respect] as is appropriate, beginning from the precise consideration of its effects to arrive at awe and praise of the Creator. That awe is followed by a stable communion of sentiment with God through confidence and hope” (as translated in Brague, “Is Physics Interesting?,” 78).

As noted above, St. Bonaventure and St. Thomas sometimes name “natural philosophy” as a genus encompassing the three speculative sciences. Natural philosophy in this general sense St. Bonaventure considers to be a vestige of the Trinity:

For all of philosophy is either natural, rational, or moral . . . . The first, or natural philosophy, is divided into metaphysics, mathematics, and physics. The first deals with the essence of things; the second with numbers and figures; and the third with natures, powers, and diffusive operations. Therefore, the first points to the First Principle, namely, the Father; the second points to the image of the Father, namely, the Son; and the third points to the gift of the Holy Spirit. (Itinerarium, III.6, 93)

Furthermore, as St. Thomas teaches, “in sacred doctrine we are able to make a threefold use of philosophy,” including natural philosophy, namely, to demonstrate certain preambles of the faith, to illuminate truths of the faith by certain likenesses or comparisons, and to dispute those “who speak against the faith, either by showing that their statements are false, or by showing that they are not necessarily true” (On Boethius’s De Trinitate, q. 2, a. 3, c.). This does not mix the water of philosophy with the wine of theology but “[changes] water into wine” (ibid., ad 5). The consideration of creatures found in natural philosophy, among the other parts of philosophy and the sciences, is also useful for building up the faith, “because through meditating on [God’s] works we are able somewhat to admire and consider the divine wisdom,” and such a “consideration leads us to admire the sublime power of God, and consequently begets in men’s hearts a reverence for God,” while also “[inflaming] the souls of men to the love of the divine goodness” and “[bestowing] on man a certain likeness to the divine perfection” through a knowledge of creatures (Summa contra Gentiles, II.2). 

This usefulness as apologetic fire and contemplative light, under the sapiential guidance of sacred theology, is just as, indeed even more, available to us today as it was to those living in ages past. For instance, the philosophy of nature helps us to articulate the integral wholeness and goodness of the created order, in contrast to an understanding and use of science which promotes powerful new technologies upon the basis of a reductive, materialist account of the natural world. Further examples are found in how the human person, a soul-body composite, is studied by the philosophy of nature. Such a study is a part of philosophy’s task of self-knowledge; it informs our philosophical reflection upon what it means to be a person and how the body is good. It helps us to see how our intellectuality exceeds mere materiality by familiarizing us with embodiment and embodied activities and prompts us to consider how, after death, we are incomplete without our bodies and how natural is our desire to be whole again, even as true hope for resurrection is in fact supernatural: “The coming resurrection of the dead is beyond the hope of nature, but not beyond the hope of grace” (St. Thomas, On the Power of God, q. 6, a. 2, ad 4; see also Commentary on First Corinthians, ch. 15, lect. 2, n. 924). The philosophy of nature, acquired among an array of intellectual habits, aids in the sapiential contemplation of creation revealed in such rich detail by the natural sciences. It helps us to see a bit more fully how divine wisdom has “ordered all things in measure, and number, and weight” (Wis 11:20).

Last, but far from least, the philosophy of nature ministers to theology’s contemplation of the person of Christ. His human nature, just like ours, was created in God’s image and likeness, for “if you ask how the Word is man, it must be said that he is man in the way that anyone is man, namely, as having human nature” (St. Thomas, Commentary on John, ch. 1, lect. 7, n. 172). St. Thomas sees this role of the philosophy of nature mystically signified in Pilate’s inscription upon Christ’s cross at Calvary:

The Hebrew tongue signified that theology and philosophy ought to be ruled by Christ; which is signified by Hebrew, because knowledge of divine matters was entrusted to the Jews. The Greek signified that Christ was to rule over the philosophy of nature and philosophy, for the Greeks were engaged in speculation about nature. Latin signified that Christ will rule over practical philosophy, because moral speculation especially flourished among the Romans. And so, all thought is brought into captivity and obedience to Christ (2 Cor 10:5). (Commentary on John, ch. 19, lect. 4, n. 2422)

To resolve to know nothing but Christ (1 Cor 2:2), therefore, is not opposed to the contemplation of His creation by the philosophy of nature. Indeed, the fullness of contemplation found in the philosophy of nature, sublimated by the light of sacred theology, complements our desire to know Him through whom all things were made.