Essentialism seems to me to be more general than anti-reductionism. One can be a reductionist essentialist.

I take essentialism to be the view that there is a privileged classification scheme that applies to all things in the world, and that the position that a thing T occupies in this scheme is entirely determined by whether T possesses a certain package of essential qualities. All other qualities of T are accidental.

Thus, to determine whether T is an X, you just need to run down a checklist of qualities x_1, . . ., x_n and see whether T has all of them. The classification scheme is unambiguous; if T doesn't have one of the x_i's, then T is an altogether different kind of thing.

Furthermore, these essential qualities aren't exceedingly exotic. Having every arrangement of atoms be a different essential quality wouldn't count. The classification scheme that the essential qualities induce is well-approximated by the classification scheme that we use in everyday life. For example, the true classification scheme should include categories that closely align with our concepts of living vs. nonliving and male vs. female.

In practice, this means that things are best thought of as a bundle of essential qualities, plus a diff of accidental qualities. Good practical and ethical reasoning focuses far more on the essential qualities than on the accidental ones.

But taking this view doesn't rule out being a reductionist. You could be an essentialist while still believing that to have essential qualities x_1, . . ., x_n means to have the arrangement of your atoms be in a certain set of possible arrangements. You would just hold that, for whatever reason, the possible arrangements of atoms can be classified with a scheme of the above sort.

Eliezer's sequence on words is a good antidote to this kind of thinking.