Selection from Etienne Gilson, The Christian Philosophy of St. Thomas Aquinas, (New York: Random House, 1956), 35-36.
[The question of the real distinction] goes directly to the very heart of the act-of-being. To posit such an act, without other determination is to posit it as absolute, since it is wholly act-of-being, and it is also to posit it as unique, since nothing can be conceived as being, which the pure act-of-being is not. If we are speaking of this act-of-being, no problem of essence and existence can arise. It is what we shall later call God. But the existing beings here under consideration are of a different sort. They are, as we have said, concrete substances, objects of our sense experience. None of them is known to us as a pure act-of-being. We find each distinguished from the others as being 'an existing animal,' or 'an existing man.' This specific determination of the acts of existing, by their forms which place each of them in a definite species, is precisely what we call their essence. Now, in the case of such beings, the only ones we know empirically, the problem of their existence challenges our thinking. Whether the pure act of existing exists or not, we do not know at this stage of our investigation. But it is at least clear that if such a being exists, it exists somehow or other in its own right, as one whose very essence is to exist. But it is quite otherwise with a tree, an animal, or a man. Their essence is to be either a tree or an animal or a man. In no case is it their essence to exist. The problem, then, of the relation of the essence to its act-of-being (esse) arises inexorably about every being whose essence is not to exist.
Such also is the so-called distinction between essence and existence, which it would be better to call the distinction between essence (essentia) and the act-of-being (esse). It cannot be doubted that this distinction is real, but it arises in the metaphysical order of act and potency, not in the physical order of the relation of parts within a material whole. This distinction is real in the highest degree, since it expresses the fact that a being whose essence is not its act of being has not of itself the wherewithal to exist. We know from experience that such beings exist, since they are all we know directly. They exist therefore, but we know too that they do not exist in their own right. Since their lack of existential necessity is congenital, it is with them as long as they endure. So long as they exist, the remain being whose existence finds no justification in their own essence. It is this that is the distinction between essence and the act-of-existing. And because it is profoundly real, it poses the problem of the cause of finite existence, which is the problem of the existence of God.