Thinkers in the Catholic intellectual tradition, including Aristotle and Aquinas, hold that substances, such as human beings or fire, cause changes in the world by acting through active qualities which are referred to as causal powers. A fire, for example, burns cotton through its active power of heat and a builder builds a house through his power to build. It is obvious, however, that in addition to acting through his power to build, the builder also uses several tools when he builds, such as a hammer and a saw. It also seems reasonable to claim that not only did the builder build the house, but that the saw also built the house since it cut all of the lumber. However, saws do not have a power to build houses as the builder does. To account for these aspects of experience within their theories of causation, ancient and medieval thinkers developed the notion of an “instrumental cause.” Such causes are also sometimes referred to as “assisting causes.” Instrumental causes are causes that are employed by another higher cause, which is referred to as a “principal cause,” to reach the higher cause's goal.
The topic of instrumental causes might initially seem like an arcane one relevant only to academics specializing in theories of causation. However, the topic of instrumental causation is very important to central issues in theology. Many in the Catholic intellectual tradition believe that the sacraments are instrumental causes used by God to cause grace and that creatures in general should be considered as instrumental causes with respect to God when they cause new effects in the created world. There were some interesting debates about the precise details of instrumental causation among medieval and early modern figures. For the sake of simplicity in introducing the topic, what follows focuses on Thomas Aquinas's views of instrumental causes.
Aquinas’s views on instrumental causation can be summarized in three key theses:
(1) Dual action thesis: Instruments have a two-fold action, one of which is proper to the instrument according to its own inherent causal powers and another action which goes beyond that which the instrument’s own native powers can achieve. For example, the saw does both the action of cutting and the action of building a house. The action of cutting is done by the saw through its own quality of sharpness. However, the action of building the house goes beyond the saw’s native powers since it does not have of itself a power to build a house.
(2) Moved-mover thesis: The instrument operates toward this second action (i.e. the one which is beyond its own power) only in so far as it is moved by the principal cause. The saw, for example, only acts toward building a house in so far as it is wielded by the builder and moved by him. If the builder were not to move the saw, it would not be able to make a house.
(3) Borrowed power thesis: Through being moved, the instrument receives a transitory power from the principal cause which enables it to work toward the second action. This is to say that when the builder moves the saw, the saw receives in itself the builder’s power to make houses. The idea of the house that exists in the builder's mind is in a way in the saw as the builder moves the saw according to the pattern of his idea. The builder's power is in the saw in a transitory way, rather than in a stable way like the saw’s own sharpness, because once the builder stops moving the saw, the saw no longer can build houses.
In addition to these three main theses about instrumental causes, it is important to note a few more key points.
First, instruments genuinely do contribute to the actions of principal causes by their native powers. Though the saw has no power to make houses of itself, it does genuinely contribute in an active way to the building of the house by its native power to cut. Without a tool to split wood, the builder cannot make a house. The action of house making occurs through the action of cutting that arises from the saw’s power. In every case of instrumental causality, the second, higher action (e.g. house building) depends on the first action (e.g. cutting). As already mentioned, the instrumental cause does not of itself have the power to achieve the second result which comes about through its action. An instrumental or assisting cause, by definition, operates toward an end which goes beyond its own nature. The way the instrument is able to do this is by being moved by the principal cause. This brings us to the next key point.
Second, Aquinas does not think that every instrumental cause is moved by its corresponding principal cause through a corporeal motion just as artificial tools are moved by craftsmen. The king, for example, moves his servant by giving him a command and the rider moves the horse by presenting it a carrot. In every case of instrumental causality, however, the principal cause causes a motion in the instrumental cause. In Aquinas’s view, the motion which the principal causes in the instrument is what enables it to work toward the end which surpasses its native power. By moving the instrument, the principal transfers a power to it. As we saw, the instrument only has the power which it receives from the principal as long as it is being moved by the principal. Instruments function as causal intermediaries between the principal and its action. The instrument takes on the principal’s power and the principal operates through it. However, there is one final important point to emphasize.
Third and finally, the instrument is not merely a passive medium through which the principal cause’s power flows. The instrument contributes to the action which the principal accomplishes through it with its own native powers. For example, the act of house building happens in virtue of cutting. Cutting is an action done through the instrument’s own native powers. A principal cause chooses a particular sort of instrument, e.g. a saw as opposed to a hammer or heat as opposed to wetness, because that instrument’s own particular powers are suitable for performing the action in virtue of which the principal cause’s action is carried out.
The notion of instrumental causality is very important to Catholic theology, and even to the faith of ordinary believers, since it offers a way to balance the primacy of divine causality, while also preserving the causal efficacy of creatures and the sacraments. Just as the builder acts through the saw both by infusing his power into it and by employing the saw’s native powers, so too does God act through creatures to cause events in the course of nature and through the sacraments to cause grace. Just as the saw genuinely contributes to the housebuilding, so too do creaturely powers contribute to causing the effects that God wills in the created order. An instrumental cause is a cause that depends on a higher cause to achieve an effect which lies beyond its power, while also genuinely contributing with its own native power. Catholic theologians think that this notion of a truly active, but yet dependent cause, captures well the causal relationship between God and creatures.
Gloria Frost is Professor of Philosophy at the University of St. Thomas in St. Paul, Minnesota.
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