The thesis that change involves the actualization of a potential tells us how change is possible, contra static monism. But how does change ever actually occur? That is to say, what is it that does the actualizing when a potential is in fact actualized? It can't be something merely potential that does it, precisely because it is merely potential. For example, potential heat cannot melt an ice cube. Only actual heat can do so. In general, if some potential is actualized, there must be something already actual which actualizes it.

This is the fundamental formulation of what is sometimes called the principle of causality. The principle is also sometimes formulated as the thesis that whatever is contingent has a cause, or the thesis that whatever comes into being has a cause. But these are really just applications of what I have called the more fundamental formulation, for a contingent thing or a thing that comes into being requires a cause precisely because its existence depends on certain potentialities being actualized. (Note that the claim that “everything has a cause” is not an application of the fundamental principle. That is a straw man that no Aristotelian or Thomist endorses, and indeed, Aristotelian-Thomistic metaphysics denies that everything has a cause. What is purely actual not only need not have a cause but cannot have one, precisely because it has no potentials which could be actualized.)

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A corollary of the principle of causality is the principle of proportionate causality. This is the thesis that whatever is in an effect must in some way preexist in the total cause of that effect. Otherwise there would be some potential in the effect that was actualized without something already actual doing the actualizing, contrary to the principle of causality.

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Sometimes this principle is objected to on the grounds that there seem to be cases where the cause lacks what is in the effect, e.g. someone can get a black eye from a person who doesn’t himself have one. But this objection simply ignores the fact that what is in the effect can be in the cause in several ways, not merely in a straightforward “formal” way. Sometimes it is suggested that evolution is a counterexample to the principle, but that this is not the case should be obvious from the fact that evolutional changes are never treated in biology as if they simply arose from nowhere – which would violate the principle – but, on the contrary, are explained by reference to preceding factors such as genetic mutations, environmental changes, selection pressures, and the like. (More on this in a later chapter.) Or consider the debate between dualists and materialists in the philosophy of mind. Materialists often argue that there cannot be any immaterial aspect to the mind, on the grounds that no such aspect could have arisen out of purely material evolutionary processes. Many dualists argue that since, as they hold, there are immaterial aspects to the mind, there must be further factors to the mind’s origin beyond the purely material ones allowed by materialists. Both sides implicitly suppose, however, that if there are immaterial aspects, then they would have to have come from something other than the purely material factors admitted by materialism. Hence both sides implicitly presuppose the principle of proportionate causality. (For more on the principle of proportionate causality, see Feser 2014b, pp. 154-59.)

The principle of causality and the principle of proportionate causality have to do with what Aristotelians call efficient causes, where an efficient cause is what brings something into being or altersit in some way. This is to be distinguished from a final cause, which is the end, goal, or outcome toward which something is directed or points. For example, an acorn “points to” or is “directed toward” becoming an oak. The phosphorus in the head of a match “points to” or is “directed toward” the generation of flame and heat. Ice “points to” or is “directed toward” being melted when heat is applied to it. And so forth.

The Aristottelian-Thomistic metaphysician holds that efficient causality is unintelligible without final causality. Efficient causality is manifest in causal regularities. Plant an acorn, and what will grow from it is an oak, not a rose bush, or a cat, or a Volkswagen. Strike a match, and it will generate flame and heat rather than turning into a snake or a bouquet of roses. Leave an ice cube out in the sun, and it will melt into a puddle of liquid water rather than turning into a stone or into gasoline. Of course, these effects might be blocked. The cause may be damaged in some way, as when an acorn is crushed underfoot or eaten by a squirrel or a match is soaked in water. Or a triggering factor that is needed if the cause is to produce its effect may be absent, as when a match is kept in a drawer instead of struck, or an ice cube is placed in the freezer rather than out in the sun. But it remains true that had the causes been undamaged and the relevant triggering factors been present, then the usual effect would have followed.

That an efficient cause A reliably produces a particular effect or range of effects B, rather than C, or D, or no effect at all, is intelligible only if generating B is the final cause of A – that is to say, if the generation of B is the end, goal, or outcome toward which A “points” or is “directed.”

Otherwise causes and effects would be “loose and separate” (as Hume would put it) and there would be no reason why A should not be associated with completely random and unpredictable effects rather than the regularity that we in fact observe. A Humean account of causality, on which there is no objective rhyme or reason to the causal order but only the regularity that the mind creates and projects onto the world, is inevitable if final causality is abandoned. Indeed, from the Aristotelian-Thomistic point of view, the early moderns’ abandonment of final causality was a key factor in the development of thinking about causality that culminated in Hume.

Aristotelian-Thomistic philosophy thus affirms a third principle concerning causality, the principle of finality, which is traditionally formulated as the thesis that every agent acts for an end, an “agent” being an efficient cause. (Contemporary analytic metaphysicians who argue that dispositions or causal powers exhibit a kind of “physical intentionality” or “natural intentionality” insofar as they are directed toward certain characteristic manifestations have essentially rediscovered the principle of finality. Cf. Place 1996; Heil 2003, pp. 221-22; Molnar 2003, chapter 3.)

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Edward Feser, Aristotle’s Revenge: The Metaphysical Foundations of Physical and Biological Science (Neunkirchen-Seelscheid: Editiones Scholasticae, 2019), 32-36.