Since, generally speaking, objections to the thesis of the real distinction stem from misinterpretations of it, a correct understanding of what is involved is very much in order.
a) First, what is this being whose constituent principles are said to be distinct? It is the concrete substance, the actual existent, and not the mere possible. Our speculation bears not on a notion but on a reality.
b) In this reality we distinguish two aspects, two principles. One is the subject or essence, the transcendental "thing" (res). The other, for want of something better, we may call "existence." St. Thomas himself refers to the latter principle indifferently as the "to be as such," "the act of being," and (our more familiar) "existence" - respectively, ipsum esse, actus essendi, existentia. We say that between these two, between essence and existence, lies a real distinction. We mean that the distinction is not merely in the mind, not just a product of reason, but that it is embedded in reality, a mark of its structure. Yet a caution must be sounded. Essence and existence are really distinct, but they are not pre-existent things which come together to form a third. In the world of creatures, before being there is neither essence nor existence, entities which are absolutely incapable of standing alone. Neither essence nor existence exists apart - even to say this jars the sensitivity. The only existent is the being they compose, saving their identity in God. Once for all, essence and existence are correlative principles having no reality except as complement of each other.
c)But how, more precisely, does one complement the other? On this point St. Thomas and his tradition have never hesitated; essence and existence serve each other (or, are related) as potency and act respectively.
Existence (esse), as such, is all act, and for any given being it is said to be the ultimate act or perfection - ultimate because, as St. Thomas remarks, "to be is the actuality of all acts, and for this reason it is the perfection of all perfections." 7 It is also, though ambiguity haunts the expression, what is most "formal" with respect to all that is in a thing; which (to forestall the ambiguity) does not mean it is a form.8
If existence is act, essence represents potency, a real capacity for act. It is not, however, the same kind of potency as prime matter; for in its own order essence is, as matter is not, something actuated or determinate. The difference shows well in spiritual substances, in which there is indeed potency but not prime matter. If St. Thomas speaks of the "matter" of such substances, he means their essence, which is comparable to prime matter because, like the latter, it too is in potency, though not in quite the same way. The spiritual essence is in potency to existence, but as essence it is all act. That is why St. Thomas describes it as "actual being of a kind, existing in potency." 9 In their own order then, both essence and existence function as determining principle; but existence, and not essence, is ultimate act, last perfection.
d) The potency of essence, we have said, is a real capacity for existence. But essence does not receive existence in the way substance receives a new determination through an accident. Existence is not, properly, an accident, a supplement to an already constituted being. Rather, existence is what is most radical, most vital to all being, and essence is what limits and determines the existence. Much more, no doubt, could be said to clarify the distinction of essence and existence. The fundamentals, however, have been gathered in the foregoing paragraphs, which we conclude with a word of warning. The translation of essence and existence into potency and act represents a special application of the potency-act formula, an application in which, under pain of going astray, the analogical character of these principles has constantly to be watched.