This from a reader:
From an Aristotelian-Thomistic point of view, the possible worlds analysis of essence has things backwards: we need to know what the essence of a thing is, before we can know what it would be like in various possible worlds; talk of possible worlds, if legitimate at all, must get explained in terms of essence, not essence in terms of possible worlds ( Aquinas, iBooks edition, page 90).
I think the modal characterization will be a dead end for us.
Two points. First, I do not understand how one could characterize the essential versus accidental distinction except modally. Second, a modal characterization need not be in terms of so-called 'possible worlds.' One should not suppose that a characterization is modal if and only if it is in terms of possible worlds.
First point first. I am a blogger and a native Californian. I might not have been either. So being a blogger and being a native Californian are accidental properties of me. I could have existed without possessing these properties. But I could not have existed without being human. So being human is an essential property of me. Generalizing, if P is an essential property of x, then x must have P, it cannot not have P. If P is an accidental property of x, then x need not have P, it could lack P. And conversely in both cases.
Note that I had to use modal words to characterize the distinction: 'might,' 'could,' 'must,' 'need not,' 'cannot.' I conclude that the accidental-essential distinction is irreducibly modal: it cannot be made except modally. It is indeed essentially modal!
To appreciate this, consider the first two accidental properties I mentioned. I was not always a blogger: speaking tenselessly, there are times at which I am not a blogger. But I was always and will always be a native Californian. Speaking tenselessly again, there are no times at which I am not a native Californian.* It follows that we cannot define an essential (accidental) property of x as a property x has (does not have) at every time at which it exists. The distinction cannot be made in temporal terms; one needs to employ modal language.
If a thing has a property essentially, then it has the property at every time at which it exists. But not conversely: if a thing has a property at every time at which it exists, it does not follow that it has the property essentially. So again it should be clear that the distinction in question is ineliminably modal.
I should make it clear that the modality in question here is non-epistemic/non-doxastic. Suppose Tom died an hour ago, unbeknownst to me. I ask you, "Is Tom teaching now?" You say, "Could be!" But of course it can't be that he is teaching now if he is dead now. You are not saying that it is (really) possible that he be teaching now; you are saying that his teaching now is logically consistent with what you know or believe, that it is not ruled out by what you know/believe.
Second point second. From what I have written it should be clear that we don't need the jargon of possible worlds to talk modally. But it is a very useful and graphic way of talking. Accordingly,
D1. P is an accidental property of x =df there are possible worlds in which x exists but does not instantiate P.
D2. P is an essential property of x =df there are no possible worlds in which x exists but does not instantiate P.
We can add a third definition:
D3. P is a necessary property of x =df there are no possible worlds in which x exists but does not instantiate P, and x exists in every possible world. Example: Omniscience is a necessary property of God: he has it in every world in which he exists, and, since he is a necessary being, he exists in every world. Non-theological example: Being prime is a necessary property of the number 7: 7 has it in every metaphysically possible world in which it exists, and it exists in every such world.
The above definitions do not sanction the reduction of the modal to the non-modal. For modal terms appear on both sides of the biconditionals. Nor could we say that the right-hand sides explicates or analyzes the left-hand sides. So I agree with Feser as quoted above. What is first in the order of metaphysical explanation is a thing's being essentially thus and so or accidentally thus and so. We can then go on to represent these states of affairs in possible worlds terms, but we need not do so.
Jenner and Dolezal. Is Jenner essentially male? I should think so. Being male is a biological determination. It can be spelled out in terms of sex chromosomes. They are different in males and females. Jenner as he is today is a sort of super-transvestite: he is not just a male in women's clothing, but a male who has had his body surgically altered to have female anatomical features. But he is still male. How could he be a woman? You can't be a woman without first being a girl, and he was never a girl.
If you deny that Jenner is essentially biologically male, will you also deny that he is essentially biologically human? If not, why not? If literal sex change is possible, is species change possible?
Is Rachel Dolezal essentially Caucasian? Well, of course. Race, like sex, is biologically based. It is not something you choose. Nor is it a social construct. Barack Obama thinks that we Americans have racism in our DNA. That's bullshit, of course. There is nothing biological about being a racist. But there is something biological about race. You can be a traitor to your country, but not to your race.
Biology matters! And so does clear thinking and honest talk. Obama take note.
*Ignoring the fact, if it is a fact, that I existed pre-natally. If this wrinkle troubles you, I can change my example.