Aquinas on Being, Chapter 2, “On Being and Essence II,” Kenny argues that that there are two types of existence—specific existence and individual existence. Here’s an excerpt:
Existence itself… can be attributed in more than one way. When we use ‘exists’ in a way corresponding to the English ‘there is a’ or ‘there are’ construction, we are saying that there is something in reality corresponding to a certain description or instantiating a certain concept—for instance ‘black swans exist’ or ‘there are plants that devour insects’. We might call this ‘specific existence’; it is the existence of something corresponding to a certain specification, something exemplifying a species, for instance, such as the insect-eating plant. But when we say ‘Julius Caesar is no more’ or ‘Julius Caesar no longer exists’, we are not talking about a species: we are talking about a historic individual, and saying that he is no longer alive, no longer among the inhabitants of the universe. We might call this ‘individual existence’ by contrast with the specific existence considered earlier. (Page 42)According to Kenny, the Thomistic Proof does not hold for either notion of existence. In case of specific existence, he says that essence and existence must be as distinct in God as they are in every other thing or the Thomistic position does not make any sense. The claim that essence is distinct from its specific existence amounts to saying that we know what a centaur is but we don’t know whether there is an x such that it is a centaur. Even if we assume that Aquinas had the notion of individual existence in mind, then his doctrine fails. Kenny writes:
It can certainly be argued that individual existence is essential to God in a way in that it is not in the case of creatures. Animals may die, and mountains may be swallowed up in an earthquake; but God cannot cease to exist. Whatever Hume may fantasize, a God that could cease to exist would not be a real God. Furthermore, a being, however grand, that had come into existence at some time in the past would not be God. If there is ever a God, there is always a God… However, the fact that everlasting existence is an essential attribute of Godhead does not mean that there is, in fact, a God. (Page 44)But if this is the case then essence and existence will be identical not only in God but in everything else.
The difficulty now is that the doctrine seems to apply to creatures as well as to God. For what are we to make of the distinction between existence and essence in creatures? Can we say that Fido’s essence and Fido’s existence are distinct? If a real distinction between A and B means that we can have one without the other, then it seems that the answer must be in the negative. For a dog to continue to exist is simply for it to go on being a dog, and for a human being to continue to exist is for it to go on possessing its human nature or essence. (Page 45)