John Locke is often seen as the father of anti-essentialism [fixed from earlier typo]. Essentially, the view is that nothing has properties necessarily in themselves or contingently in themselves. When you think of me as a human, being rational is essential, but when you think of me as an animal it isn't. When you think of me as a husband, being married is essential. When you think of me as a human being, it isn't. Locke's way of putting this is that my properties are only essential to my being me insofar as you're thinking of me under a particular sortal term.
Locke's reasoning on this is fallacious, as Saul Kripke showed in his famous Naming and Necessity (which didn't really say much more than Leibniz had already said in his preface to his commentary on Locke's Essay Concerning Human Understanding, the New Essays on Human Understanding).
I had a conversation this morning that made me wonder if Locke was really the anti-essentialist [fixed from earlier typo] he's usually portrayed as. It seems to me that the general point Locke was making isn't so far removed from something Aristotle and Aquinas make in their unmoved mover arguments. The unmoved mover argument gets commonly misunderstood as a mere causal argument. If A is caused by B, then we ask what caused B. Then we turn to C, which in turn was caused by D. Hume rightly objects to this argument for an unmoved mover by pointing out that each bit is explained if there's an infinite series of such moved movers. If that's all the argument is, then there's no reason to conclude that there's an unmoved mover unless you can show a problem with an infinite past, which mathematics can make sense of since Leibniz and Newton.
That's not what Aristotle and Aquinas meant, however. Their notion of a cause makes what Hume thought of as a cause seem quite anemic. They were really talking about a dependence relation that involves notions of contingency and a more robust notion of explanation. A is contingent in the sense of requiring something else entirely for its very existence. So is B. It's true that B explains A in some sense, but B isn't an explanation for A unless B is itself explained. No contingent thing that didn't have to exist has its explanation in itself, and since everything we encounter in daily life in space-time is a contingent thing, then all our prime examples of things will turn out to be contingent things. What Hume failed to see is that once you're thinking in these terms, it's quite clear that providing an explanation for all the contingent things in terms of each other still doesn't count as providing an explanation for why there are any contingent things at all. That's why Aristotle and Aquinas saw there still to be a need for something beyond intermediate causes. It's not a necessary explanation unless it's got a necessary cause, without which it wouldn't exist. (None of this requires sufficient explanations, as Leibniz and Kant's versions of the argument required.)
Now what does this have to do with Locke? It seems to me that Locke is thinking along the lines of the people Hume criticizes here. He wants to say that nothing in itself is necessary or had to have the properties it has. Only when thinking of it under a sortal do you get the distinction between necessary and contingent, because the things are necessary or contingent given that property you're thinking of as definitional. When ignoring that, you get different properties. This is all very similar to the notion of an intermediate cause like that of Aquinas or Aristotle, in which something is what it is only because of things outside of it. It's a different issue, but it raises similar questions.
Now why do I think this should lead us to question Locke's anti-essentialism? There's one sort of being, which the unmoved mover argument is supposed to lead us to (and Locke does give his own particularly bad version of the cosmological argument in Book IV that makes all the mistakes Hume complains about that Aquinas and Leibniz were more careful about) who does seem to have properties essentially. If Locke admits that God has properties essentially, including existence, which I think is required for his own argument in Book IV, then aren't there some properties that are had essentially and not just according to a sortal?
Maybe Locke can get out of this, but it seems to me that he'd have to work hard to explain God's necessity if nothing can have properties essentially or accidentally except when thought of under a sortal.