Philosophy of Nature: III. The Four Causes
C: Efficient Cause
In addition to matter and form, Aristotle and Aquinas after him, shows that there must also be an active source for any motion or change. While change, both accidental and substantial, is the process of some new actuality, form, coming to supplant the lack of that form (privation) in something in potency to the form and which persists through the change (matter), there must also be something that makes the potential actual and brings about the change in question. This is what Aristotle and Aquinas call the agent or efficient cause.
From what has been said, therefore, it is clear that there are three principles of nature, namely matter, form and privation. But these are not sufficient for generation. For what is in potency cannot reduce itself to act: for example, the bronze which is in potency to a statue does not make itself [to be] a statue, but it needs an agent to draw out [extrahat] the form of the statue from potency to act. Neither can the form draw itself from potency to act. I mean the form of the thing generated which we say is the term of generation, because the form does not exist except in that which has been made to be. However, what is acted upon [operatur] is in the state of coming to be, i.e., as long as the thing is being made. Therefore, there must be, besides the matter and form, some principle which acts. This is called the efficient, moving, or agent cause, or that from which there is a beginning [principium] of motion.[1]
Before coming to be, the thing to be generated is only potential. It cannot, therefore, give itself the actuality it will have as a result of being changed. There must be something actual beyond what comes to be to make the potential actual. The efficient cause initiates or sustains the change a thing undergoes from a being in potency to a being in act. It brings about that what was initially potential is now actual, and without some cause to play this role, potencies would not become actualized. Statues don’t just sculpt themselves, and water does not become hot without some cause heating it. Efficient causes provide the actuality to make the potential actual.
An efficient cause may be external, as in the case of accidental changes as, for example, a sculptor is the efficient cause of a statue, or the action of fire causes water (and the pot it is in) to become hot. In cases of accidental changes, the agent or efficient cause brings about the accidental effect in some substance that continues to exist: shape in bronze, or heat in water. The efficient causes of substantial changes are similarly external to the things that comes to be: fire is also the efficient cause of wood changing into smoke and ash; parents are the efficient causes of coming to be of offspring.
The efficient cause of a certain accidental motions that are natural to a material substance, however, may also be internal to the thing, though the agent cause is still extrinsic to the effect it brings about, even if the effect is within the agent (as especially in the case of animals which are ‘self-movers’). In such natural motions, the efficient cause of the motion is the natural form of the thing that changes. This applies to all the kinds of motions living things undertake, i.e. natural alterations (an apple turning from green to red), the growth of plants and animals, and all natural local motions. This doctrine of natural motions even applies to inanimate bodies since for Aristotle and Aquinas natural bodies tend to their natural place (fire tends up, earth tends down). The internal source for the motion of growth for all living things, as we will see in the next chapter, is the substantial forms of animate substances, i.e., their soul. Animals have the further motions of locomotion and sensation, and the efficient cause of these motions is also the soul.
The fact that motions are actualizations caused by an agent, something which is itself actual, leads to some general principles Aquinas invokes in various contexts.
1. Principles of Causality. In general, every effect that comes about (i.e., change, event, happening) requires a cause that brings it about, and provides an explanation for it.[2] The existence of the effect, then, at least implies and gives knowledge of the existence of its cause. (This principle is the basis of quia demonstrations which we considered in Chapter 1 – The Basics of Logic.)
a. Whatever is moved is moved by another. Since motion is the process of reducing a thing’s potency to act, something in act must cause every motion or change, and so the cause must be other than what is in motion. As Aquinas notes, “potency does not reduce (educit) itself to act; it must be reduced to act by something that is in act.”[3] Thus, nothing causes its own process of change. The cause of a thing’s motion must be something other than what is moving.
The principle is true, incidentally, even of animals, which seem at first glance to move or change themselves; for what this always amounts to is really just one part of the animal being changed by another part. A dog “moves itself” across a room, but only insofar as the potential for motion in the dog’s legs is actualized by the flexing of the leg muscles, and their potential for being flexed is actualized by the firing of the motor neurons, and the potential for the motor neurons to fire is actualized by other neurons; and so on.[4]
b. Every agent acts insofar as it is in act. Viewed from the side of the efficient cause, every agent must be actual to bring some potency to actuality. The agent causes in virtue of what is actual, not potential, in it.
c. Nothing causes except by an act which it possesses. A thing may be actual in many and various ways, but it only acts as an agent, an efficient cause, with its actuality that is appropriate to the effect it is to bring about, one that reduces the potency of the patient or what is moved to actuality. My thinking is an actuality I possess, and I might be actually warm, but I cannot melt ice just by thinking about it, nor without my heat coming in contact with the solidity of the frozen water.
2. Principle of Proportionate Causality. From the principle that every agent acts according to its own actuality, it follows that effects resemble their causes. From the nature of an effect one can often reason to something of the nature of the cause. However, in no case does a cause give what it does not have, or causes an effect greater than itself. But there are different ways that a cause may be actual so as to reduce the potency of another, the patient of the causal action, to actuality according as the effect more or less perfectly resembles its cause.
a. Formally – Efficient causes may bring about an effect of the same sort, and be said to cause an effect formally, i.e., to bring about an effect with the same form. But “same” here means merely to have the specifically (not numerically) same form. For example, when a hot thing, a stove for instance, heats, it causes an effect with the same accidental form as itself (heat) though in different matter. As we saw with regard to transient motions (material alterations), the very same, numerically identical heat does not leave the stove and pass into the water it heats. Rather the actuality of the stove reduces the water’s potency to be hot to actuality, and so cause and effect are hot with their own, numerically distinct heats; while stove and water come to be formally the same (hot), their heats are numerically different. Moreover, the actuality of the effect (e.g., temperature in this case) cannot exceed that of its cause. An effect cannot formally exceed its efficient cause.
b. Virtually – An agent’s active potency may actualize an effect’s passive potency, but do so without that actuality being the (specifically) same form as the effect; its actuality may be of a different sort, but this actuality still causes an effect by its active power (virtus). Not only hot things like stoves produce heat: striking flint against steel may produce hot sparks which ignite a flame, the friction of rubbing two sticks or hands together make them hot, or microwaves (in a microwave oven) can likewise heat things without being themselves physically hot (just energetic).
c. Eminently – An agent may also cause an effect that is merely analogous to itself, such as when an artist or craftsman causes an effect that does not literally or physically resemble himself, but reflects the insight or plan he conceived. In such cases, however, the effect is still not greater than its cause’s own actuality.
3. Causal Subordination – Efficient causes may bring about an effect that in turn acts as a cause, such that they form a series of prior and subsequent causes and effects. The later causes/effects in such series may be subordinated in two ways:
a. Incidental (per accidens) subordination: wherein the agent causes an effect to be but not to act. The causality of the prior agent does not continue in the causality of its effects.
- The prior agent’s action is limited to generation of effect. For example, man begets man, but the father’s action ends with his son’s generation.
- The removal of prior cause does not affect the being or activity of later causes. A man can die and his son can still generate his father’s granddaughter. The son has his own generative power even after the passing of his own father.
- This sort of causal series can be characterized as a horizontal, or a succession in time. In a temporally successive series of caused causes (i.e., of effects which are themselves causes of the same sort), the series causality can stretch back indefinitely. Because he view the order of material causes in this way, Aristotle concluded that the physical universe (along with the spheres that were causing it to continue) was eternal, of an infinite past and future temporal duration.
b. Essential (per se) subordination: wherein an agent causes an effect not only to be, but to be a cause.
- Prior agents in such essentially subordinated series cause their effects themselves to act as causes which bring about their own effects after them. For instance, when a hand moves a cue stick which moves a billiard ball; one train car pulls another until the last pulls the caboose; or in a chain under tension, each link whatever its position (n-1) supports the link after it (n). The movement of the stick, the motion of subsequent train cars, and ability of prior links to support later ones, all depend on the causal activity of the prior causes. Cue sticks, train cars, and chain links only move or support later members of such essentially subordinated series because they themselves are moved or supported by a prior member.
- Since the causal efficacy or change of the later members depend on the prior, the latter’s activity of being a cause is simultaneous with the causality of the prior. Each subsequent effect acts as a cause by transferring causality it receives from its proximately prior cause. E.g., the movement of train car 23 causes train car 24 to pull train car 25, which then pulls the last car (the caboose).
- The removal of prior cause removes the ability of the subsequent effects to act as causes. If the hand stops moving the stick, the stick does not move the billiard ball; the stoppage of a prior train car brings all those behind it to a halt; and an unsupported or disconnected chain link falls or stops pulling all those that had depended on it.
- If such an essentially subordinated series of caused causes is actually and actively transferring causality, it cannot be infinite. Rather, the simultaneous causal series must have a finite number of members, for without a first generating or foundational cause, there is no causality to be transferred.
First Cause of Motion, Efficient Causality, and Existence
Aquinas uses the notion of essential subordination of efficient cause in various ways in the first three of his five proofs for the existence of God.[5] The basic line of reasoning claims that we observe effects that must have been produced by some efficient cause. That cause itself is either caused by something prior, or it is uncaused. Since there cannot be an infinite regress in essentially subordinated causes (i.e. causes that rely on prior causes to exercise their own causality), there must be some First Cause (which Aquinas believes must be God) of all the effects that follow from it. However, whereas Aristotle believes that the world has always been in motion, Aquinas believes, against the opinion of St. Bonaventure, for example, that there is no way to prove or disprove this belief in the eternity of the world from a philosophical point of view.[6] Aquinas, therefore, teaches that it is an article of faith that the world had a beginning in time.[7] Nevertheless, he still believes (as Aristotle himself did) that even an eternal material world requires a First Efficient Cause to sustain the motions that occur within it, and an analysis of the motion of a world stretching back into an infinite past, nevertheless, leads to the same conclusion that there is a God, who is the First Cause of that infinitely old world.
As we saw in Chapter 1 of this Overview, the problem of whether the eternity of the world is a philosophically necessary conclusion, and whether its truth implies the non-existence of God, were exceedingly important problems for Aquinas. So much so, that he wrote an entire work on the subject: On the Eternity of the World.
[1] On the Principles of Nature, c. 3.1
[2] Later philosophers will interpret this as the Principle of Sufficient Reason, but Feser notes that while efficient causes provide explanations, not all explanations are through causes, at least not through efficient causes. Agents cause by reducing potency to act, while sufficient reasons may explain something without any such actuality. He notes, “Thus while God is not his own cause, he is his own sufficient reason.” Scholastic Metaphysics, p. 140.
[3] SCG I, 16, no. 3.
[4] Feser, Aquinas: A Beginner’s Guide, p. 23.
[5] ST Ia, q. 2, a. 3; See Chapter 6 below.
[6] ST Ia, q. 46, a. 1.
[7] ST Ia, q. 46, a. 2.
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