Encyclopedia of Philosophy: Aquinas on Essence and Existence

Difficulties with the notion of transubstantiation do not, of course, call in question the general concepts of substance and accident outside this particular, and perhaps perverse, theological application of them. But the Aristotelian analysis of change raises other problems which Aquinas considers. If accidental change is to be understood as one and the same substance taking on various accidents, and substantial change is to be understood as one and the same matter taking on various substantial forms, should we understand the origin of the material world itself as being a case of one and the same essence passing from non-existence to existence? This was not, of course, a question which arose for Aristotle, who did not believe in creation out of nothing; but some later Aristotelians had posed the question and answered it in the affirmative. Aquinas firmly rejects the idea: creation is totally different from change, and is not to be understood in terms of existence being fastened on to an essence.