1. Act and potency
1.1 The general theory
1.1.1 Origins of the distinction
The first of the famous twenty-four Thomistic theses reads:
Potency and act are a complete division of being. Hence, whatever is must be either pure act or a unit composed of potency and act as its primary and intrinsic principles. (Wuellner 1956, p. 120)
The distinction between potency and act is fundamental not only to Thomism but to Scholastic philosophy in general (though as we will see, Scotists and Suarezians disagree with Thomists about how to interpret the distinction). It is absolutely crucial to the Scholastic approach to questions about the metaphysics of substance, essence, and causation (and for that matter to Scholastic philosophy of nature, philosophical psychology, natural theology, and even ethics). We would do well to begin, then, with an outline of the theory of act and potency. Subsequent sections of this chapter and the next will develop and defend key aspects of the theory as they apply to causation. In later chapters we will see how the theory applies to other metaphysical issues.
The theory has its origins in Aristotle's account of where the Eleatics on the one hand, and Heraclitus on the other, went wrong in their respective positions vis-à-vis change versus permanence -- an account that was extended by Scholastic writers to a critique of the Eleatic and Heraclitean positions vis-à-vis multiplicity versus unity .
Parmenides and Zeno denied the reality of change. Parmenides' position is essentially that (1) change would require being to arise out of non-being or nothingness, but (2) from non-being or nothingness, nothing can arise, so that (3) change is impossible. Zeno aimed to reduce the notion of local motion to absurdity via paradoxes some of which presuppose that traversing a finite distance would require traversing an infinite number of shorter distances. For example, in the dichotomy paradox, Zeno suggests that a runner can get from point A to point B only if he first reaches the midpoint between A and B; but he can reach that midpoint only if he first reaches the point midway between A and the midpoint, and so on ad infinitum . Hence he can never reach B, and indeed can never even move beyond A.
A natural first response to such arguments would be to apply the method of retorsion and argue that those who deny the reality of change are led thereby into a performative self-contradiction. The Eleatic philosopher has to move his lips or pen in order to put his argument forward; if he bites the bullet and denies that even his lips and pen are really moving or that he is really trying to change the minds of his listeners or readers, he still has to go through the steps of his reasoning in his own mind, and that involves change. The reality of change is not self- evident, insofar as it is not a necessary truth that any change ever actually occurs. But it is still evident insofar as we have to acknowledge it in order to argue for anything at all. (Cf. Smith and Kendzierski 1961, p. 16)
This tells us at most that something has gone wrong in the Eleatic arguments, but not what , exactly, has gone wrong. The problem with Parmenides' reasoning, in Aristotle's view, is neither in the inference from (1) and (2) to (3), nor with premise (2), with which Aristotle agrees. It is rather with premise (1), the thesis that change would involve being arising from non-being. For there is, according to Aristotle, an alternative analysis of change, on which it involves, not being arising from non-being, but rather one kind of being arising from another kind. In particular, there is being-in-act --