So, searching for J. L. Mackies "argument from queerness", I stumbled upon this blog entry.
Now, Mr. Feser seems to be a quite... controversial figure, to say the least, but please let's resist the temptation to simply dismiss him.
[...] a Euclidean triangle drawn slowly and carefully with a ruler is a good one while a Euclidean triangle drawn sloppily is a bad one; and so forth. The core idea is that of a good or bad specimen of a kind of thing, of something which more or less adequately instantiates what is of the essence or nature of the kind.
I find this usage of "good" quite weird and I am very suspicious of the claim that it is possible to get from this kind of "good" to moral goodness. But let's leave all that aside.
The fundamental question is how this "essentialism" (Aristotelian, Thomist) can ever make sense:
How to find out what the "essence" of a particular thing is?
How to find out if a particular thing approximates its essence well? It may simply have a different essence...
I've drawn the following example, which hopefully makes the problem clear:
Now what is the green figure? What is the gray figure? Does the green figure instantiate its essence, which is "being a Reuleaux-triangle", near perfectly? Or is it instantiating its essence, which is "being a circle", extremely badly? Similarly for the gray figure.
I can't be the first person who sees this very obvious and in my opinion very serious problem with postulating "essences".
There are probably counterarguments to it. Does somebody know any?