One of the points of friction between Aristotle’s natural science and modern evolutionary thinking is the idea that Aristotle is committed to essentialism whereas the theory of evolution dissolves any notion of essences. One difficulty in evaluating the claim is that “essence” is exactly the sort of term that Aristotle tried to avoid. When “essence” occurs in an English translation, it is a simplification of an obscure phrase like “the what” or “the what it is” or “the what it was to be”. Even if we say that these phrases all mean “essence” the difference in signification is important: all of Aristotle’s phrases center around the use of the word “what”, which is used (improperly) as a substantive pronoun. As a pronoun, its whole nature is understood in relation to some other nature. “Essence”, however, is a noun pure and simple, and so the mind is not forced to see it in relation to something else. Essence, as a sign, is absolute and what is relative.
When we simply plow through Aristotle’s text changing every “what is” to “essence” we will be prone to fall into exactly the sort of error that is thoroughly discredited by the insights of evolution. In Aristotle’s system, the “what”, when said of a natural thing, refers to a compositeof matter and form; when said of a being without matter thing, it refers to form alone. The first “what” is inseparable from a change and flux so far as it is defined with matter, and so if we say that “the what” means “essence” than the essence of natural things is changeable. Changeability is not the whole of its essence (for form is essential to its essence too) but it is inseparable from it. In this sense of essence, the essences of the things the natural scientist studies are changeable. The difficulty is that “essence”, because of its absolute character, cannot mix with the idea of being changeable. The mind recoils from the idea of “changeable essence” as a contradiction, or at least an extremely poor choice of words. This is why when we find various good reasons to say that essences (or what we thought were essences) are not fixed, it is better to junk the idea of essence altogether. This is fine, but even after we junk the word “essence” we are still left with Aristotle’s “the what”, which, when applied to natural things, includes matter and is therefore capable of change.
Any charge that Aristotle was an “essentialist” has to keep in mind the peculiar notion that he forged of natural essences, sc. ones with matter. This presence of matter makes natural beings essentially unfixed, changeable, and unintelligible, even though this is not the whole of what we can say about them. There is also a principle by which we can come to know natural things, even to know them as changeable and somehow unknowable.
Now there is obviously a sense in which natural things have an unchangeable essence in Aristotle’s system, sc. so far as they are natural- which for him means so far as they are changeable- their essence is to be composites of matter and form. The theory of evolution, however, takes changeability for granted and so has no use for an account of the essential principles of the changeable as such.