Aristotle does not treat explicitly of the distinction between essence and existence. This will come as no surprise when it is remembered that also not expressly considered by him are formal multiplicity and the relation of limited beings to pure act. All the same, there is nothing in his philosophy that opposes the distinction; in fact, considering on the one hand his general orientation toward the concrete, the individual existent, and his looking to essence for the intelligibility of things on the other, it may be said that the logical tendency of his thought is toward the real distinction. However, the first clear intimations of the problem come not from Aristotle but from the Neo-Platonists.
One of the earlier names to the purpose is Boethius, who, in a passage that would be much quoted in support of the real distinction, contrasts "to be" (esse) with "what is" (quod est).3 It has been argued, however, and successfully it appears, that Boethius does not use esse in the existential sense, and certainly he says nothing about the reality of our distinction.4 For that, one has to look to the (much later) Arabian philosophers, for instance Alfarabi and Avicenna, where, at last, the real distinction is unmistakably affirmed. But Avicenna, in particular, left much to be desired, for existence is conceived by him as a kind of accident to essence; which earned him the sharp criticism of his fellowArabian Averroes, and later of St. Thomas. The truth of the matter is that, notwithstanding these earlier manifestations, it remained for St. Thomas to put the doctrine of the real distinction into proper focus and to give systematic development to the consequences emanating from it. And yet, one searches him in vain for an explicit and formal justification of the doctrine, likely because the controversy surrounding it had still to be ignited. Despite that, the thesis is implied in all his philosophy, is in fact central to it, so much that if the appertaining texts are interpreted in another sense, its entire structure collapses. As for the polemical phase of the question, this takes shape after St. Thomas, when Giles of Rome, affirming the reality of the distinction in a manner far from felicitous, invited the criticisms leveled against him by Henry of Ghent. Subsequently Scotus, and still later Suarez, both denying the real distinction, would stir interminable debate and discussion.