This post defends the real distinction between essence and existence. For some background, see Geach on the Real Distinction I.
In Aquinas on Being (Oxford 2002, p. 45), Anthony Kenny writes, "Peter's continuing to exist is the very same thing as Peter's continuing to possess his essence; if he ceases to exist, he ceases to be a human being and vice versa."
What Kenny is doing in this passage and the surrounding text is rejecting the real distinction between essence and (individual) existence. Thus in a cat, a dog, or a man, there is no distinction in reality between its essence or nature and its existence. In general, for items of kind K, to exist is to be a K. Thus for Socrates to exist is for Socrates to be a man; for Socrates to continue to exist is for Socrates to continue to be a man; and for Socrates to cease to exist is for Socrates to cease being a man.
The claim that for items of kind K, to exist is to be a K, is to be understood, not as a logical or metaphysical equivalence, but as an identity that sanctions a reduction: the existence of Ks just is (identically) their K-ness. Individual (as opposed to what Kenny calls specific) existence reduces to nature. But that is just to say that there is no real distinction in a thing between its individual existence and its nature. For example, there is no non-notional or real distinction in Socrates between him and his existence.
I have three objections to this broadly Aristotelian theory of existence according to which individual existence reduces to nature.
An Argument from Contingency
Socrates might never have existed. If so, and if, for Socrates, who is a man, to exist = to be a man, then Socrates might never have been a man. This implies that a certain man, Socrates, might never have been a man, which is absurd. Therefore, it is not the case that, for Socrates, to exist = to be a man.
The first premise ought to be uncontroversial. Speaking tenselessly, Socrates exists and Socrates is a man. But there is no logical or metaphysical necessity that the man Socrates exist. So, Socrates, though he exists, is possibly such that he does not exist. (This is equivalent to saying that he is a contingent being.) So, given that to exist = to be a man, the man Socrates is possibly such that he is not a man. But this contradicts the fact that Socrates is essentially a man. For if he is essentially a man, then he is necessarily such that he is a man. Therefore, it is not the case that, for Socrates, to exist = to be a man.
Convinced? Here is another way of looking at it. I point to Socrates and say, 'This might not have existed.' I say something true. But if I point to him and say, 'This might not have been a man,' I say something false. Therefore, for Socrates, to exist is not to be a man. Of course, he cannot exist without being a man, and he cannot BE a man without BEING. But that is not the question. The question is whether Socrates' being or existence is reducible to his being a man. I have just shown that it is not. Therefore, there is a real distinction between essence and existence in Socrates.
What holds for Socrates holds for every man. No man's very existence is reducible to his being a man. And in general, no individual K's individual existence is reducible to its being a K.
An Argument from Reference
If for Socrates to exist is for Socrates to be a man, then, when he ceases to exist, he ceases to be a man. But then the proper name 'Socrates' used after the philosopher's death does not refer to a man. But it does refer. For I can make true statements about Socrates, e.g., 'Socrates taught Plato.' And the name refers to a man. When Socrates ceased to exist, 'Socrates' did not commence referring to some other thing, a jelly fish say, or a valve-lifter in a '57 Chevy, or more plausibly, a corpse. A man taught Plato, not a corpse, or a pile of ashes. Therefore, it is not the case that for Socrates to exist is for Socrates to be a man.
To understand this argument, please note that it is not being denied that, necessarily, at every time at which Socrates is alive, Socrates exists if and only if he is a man. Socrates cannot exist without being a man, and he cannot be a man without existing. What is being denied, or rather questioned, is the identification of Socrates' existing with his being a man. As I have pointed out many times before, logical equivalences do not sanction reductions.
A Third Argument
We cannot say that to exist = to be a cat, for then only cats could exist. We, or rather the Aristotelian, has to say that, for cats, to exist = to be a cat. In general, for K-items, to exist = to be a K. But why stop here? Can we stop here? There are no cats in general. There are only particular cats, any two of which are numerically distinct, and each of which has its own existence. Consider Max and Manny, two cats of my acquaintance. Each has his own existence, but they share the nature, cat. So if each exists in virtue of being a cat, then each exists in virtue of being the very cat that it is, which is to say: for Max to exist is for Max to be Max, and for Manny to exist is for Manny to be Manny. But then, generalizing, to exist = to be self-identical. The theory we began with collapses into the existence = self-identity theory.
But while each thing is self-identical -- this is just the Law of Identity -- no contingent thing is identical to its own existence. For if Max were identical to his own existence, then Max would necessarily exist. If God exists, then God is identical to his own existence. But Max is not God. Therefore, existence cannot be reduced to self-identity in the case of contingent beings.
Of course, given that contingent things exist, they must be self-identical, and they cannot BE self-identical unless they ARE or exist. But there might not have been any contingent things at all. So the existence of a thing cannot be reduced to the self-identity it could have only if it exists. Get it? If yes, then you understand the real distinction.