The distinction of being into act and potency was first enunciated by Aristotle, and while he may not have unveiled every development which this doctrine invites, no one questions that he established the essentials. St. Thomas had only to pursue the line of thought marked out by Aristotle, and the doctrine would attain fulfillment. 1
It is in the Physics, when searching for the principles that would account for change, that Aristotle seems first to have employed the notions of act and potency. Already in the first Book the co-principles of matter and form are distinguished as act (form) and potency (matter). The distinction is subsequently invoked as the key to the understanding of motion, which is defined in these very terms as the act of what is in potency as such, that is, so far as it is in potency. Finally, after various other applications of the notions, the work culminates in the prime mover, pure act in which all motion of the universe is resolved.
In the Metaphysics act and potency reappear to constitute, with the predicaments, the first divisions of being. The whole of Book Θ is given to act and potency, a book in which there begins to emerge Aristotle's concern to bring them to bear, not just an motion as such, but on a far wider range of reality, and indeed on all reality, that of pure form and immovable act included. What he does, in effect, is to lay the groundwork for the theological speculations of Book Λ, in which the primary substance is expounded under its proper characteristic, its pure actuality. However, Aristotle's utilization of act and potency is not confined to strict metaphysics; rather, the doctrine is pressed into service throughout his works. In psychology he is constantly having recourse to these two principles, and finds applications for them in logic and even in mathematics. Hence, it is really not to be wondered at that act and potency should be regarded by some as the cornerstone of his entire philosophy. But whether this be so or not, the central importance of the doctrine is clear enough.
As for its origin, that, as already intimated, traces to the problem of change. Aristotle's solution is traditionally, and quite properly, taken as middle to the extremes represented by Parmenides and Heraclitus. Between being and nonbeing Parmenides could find no alternative; the result was to deny the reality of change or becoming. His reasoning in point is simple, and decisive (granted his premise). Being, he protested, cannot come from being which already is; in other words, what is cannot become (what it already is). Nor can it come from nonbeing, which, in his view, must be nothing. And that is of course true; where there is nothing, nothing can emerge. Consequently, if being cannot come from being nor from nonbeing, it cannot come, or become, at all. Result: there is no becoming, there is only being.
To this extreme the traditional Heraclitus opposes another, that change is not only real but (to all indications) the only reality; for, behind the incessant flow of appearances no abiding principle or reality is to be found. Hence all is becoming, and being as such does not exist. But if being is denied and only becoming affirmed, even becoming would seem to be ruled out; for what possible meaning is there to a becoming that does not become something, some being? 2
The problem, then, is one of salvaging both being and becoming. The distinction of act and potency points the way - the way out of the Parmenidean dilemma. We have simply to recognize a kind of intermediate state between being as fully determined, which is being in act, and nonbeing considered as pure nothing. This intermediate state is being in potency, which is real though not yet perfectly realized. Change, accordingly, becomes possible and is explained as a transition from being in potency to being in act.3 Suppose (to illustrate) that a sculptor decides on a statue. Selecting a block of marble he carves it down to his subject. Metaphysically speaking, what has taken place? When the statue is finished, it exists in act. But where was it before the sculptor put chisel to marble? Clearly, it did not exist in act, but did it exist in any way at all? If you say: no, it had no reality whatever, then the fabrication of the statue becomes unintelligible, inexplicable - an emergence from absolute nothing, which is absurd on the face of it. In truth, the sculptor could not apply himself unless there was at his disposal a suitable material, the marble, from which he extracted, as it wem, the statue. To repeat, the statue did not exist in act in the naked marble, but it could be hewn because it was there in potency. In the fabrication it went from a statue in potency to a statue in act.
What is true in art is analogously true in nature, in the germination and growth (say) of a seed. The plant that has reached full development did not exist in act in the seed; yet exist in the seed it did, but in potency. We may generalize these examples and apply their particulars to all like cases. Change, every change, we shall find, is a going from being in potency to being in act. With that, both realities are safeguarded, the reality of becoming as well as of being. Thus what was (to some) the insoluble problem of change is resolved by the distinction of act and potency. These rudiments in hand, we have now to examine each member of the distinction more closely. 4