Fourth Way, III

St. Thomas is clear that all the Five Ways work by moving from effect to cause. The Fourth Way, therefore, argues that what is most good is the cause of what is less so. The proof would follow from two premises, both of which St. Thomas argues for: a.) the imperfect, as such, is being caused, and b.) causes are more perfect than effects. Both follow from the same principle: all imperfection is potential as opposed to actual existence. The self- evident concept at the basis of the proof is the nature of act as opposed to potency. Aristotle insists that act is simply a primary notion, set at the foundation of metaphysics. 

“Since we have treated of the kind of potency which is related to movement, let us discuss actuality-what, and what kind of thing, actuality is. For in the course of our analysis it will also become clear, with regard to the potential, that we not only ascribe potency to that whose nature it is to move something else, or to be moved by something else, either without qualification or in some particular way, but also use the word in another sense, which is the reason of the inquiry in the course of which we have discussed these previous senses also. Actuality, then, is the existence of a thing not in the way which we express by ‘potentially’; we say that potentially, for instance, a statue of Hermes is in the block of wood and the half-line is in the whole, because it might be separated out, and we call even the man who is not studying a man of science, if he is capable of studying; the thing that stands in contrast to each of these exists actually. Our meaning can be seen in the particular cases by induction, and we must not seek a definition of everything but be content to grasp the analogy, that it is as that which is building is to that which is capable of building, and the waking to the sleeping, and that which is seeing to that which has its eyes shut but has sight, and that which has been shaped out of the matter to the matter, and that which has been wrought up to the unwrought. Let actuality be defined by one member of this antithesis, and the potential by the other. But all things are not said in the same sense to exist actually, but only by analogy-as A is in B or to B, C is in D or to D; for some are as movement to potency, and the others as substance to some sort of matter. 

 Metaphysics, Book IX chap. 6

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