Reading it closely, we should find that St. Thomas' analysis of being, with its stress on the real distinction of essence and existence, scores a significant advance over the Aristotelian analysis. The result, as Professor Gilson relates in a remarkable study,11 is not simply Aristotle revisited but Aristotle revised and indeed transformed. In effect, what St. Thomas achieved was a metaphysics far more original than it has been given credit for, even by some professing his name. In this connection, it may seem a curious thing but the persistent tendency of philosophers has been, and history proves it, to construe being as nature or essence. It is plainly so in Platonic thought, and even the substance (ousia) of Aristotle has still the appearances of mere subject of being, namely essence.
Avicenna, it is true, represents a notable leap forward in thought as well as in time, only, however, to be castigated by his fellow Arabian, Averroes. Yet Avicenna still falls short of the Thomistic achievement. In his middle position existence is indeed discriminated from essence, yet essence again emerges as the basic structural element of being, upon which existence (the actus existendi) supervenes in much the manner of accident - a tacking on, as it were. In time a considerable number of Scholastics, under the influence of Scotus and Suarez, would also commit themselves more or less wittingly to the essentialist conception of being.12 The same is true of modern philosophy, possibly more so; from Descartes to Hegel, with Wolff and Kant as intermediaries, the essentialist position is in varying degree in command.
But when we take stock of St. Thomas, a new turn of thought appears. Not only does he ceaselessly affirm that in created beings existence is really distinct from essence - of which, for that matter, he was never in doubt - but also that existence is act, of all perfections of being the ultimate one, and that God himself is Ipsum esse subsistens, Subsistent Being (existence) Itself. Being, accordingly, whether of God or of creatures, is pre-eminently existence. So that it is more in keeping with the spirit of St. Thomas to say - though the converse, too, is perfectly correct - that being is an existence determined by an essence. We need not hesitate, then, to describe his metaphysics as existentialist, though not, it should be emphasized, in the sense that marks certain contemporary philosophies. Indeed, it is the existentialist character of St. Thomas' thought which sets it off from every essentialist or rationalist philosophy, ancient, Scholastic, and modern, and lends it the stroke of high originality.