The Unity of Definition

Nominal Essences

We saw in Chapter 1 that, in On Interpretation and in the Posterior Analytics, Aristotle wonders how the differentia and the genus make up a unity and questions Plato's method of division. When one is asked whether man is a terrestrial or nonterrestrial animal, the method of division leads one to answer that man is a terrestrial animal. But, asks Aristotle, “what prevents all this from being true of man yet not making clear what a man is or what it is to be a man?” Man is a terrestrial animal, but the method of division gives us no reason to think that being a terrestrial animal is part of man's essential nature. Mere truth is a necessary but not a sufficient condition for a formula's expressing an essential nature; there must be something more, but what? Some interpreters have thought that the missing element must be meaning-equivalence: that if “man” means “terrestrial animal,” then “man is a terrestrial animal” states the essence of man. But Aristotle has something else in mind. In a previous article I called his answer to this problem “the doctrine of proper differentiae.”

In the Metaphysics VII, 4, Aristotle says that “the essence of each thing is what it is said to be in virtue of itself [kath' hauto]. For being you is not being musical; for you are not musical in virtue of yourself. What, then, you are in virtue of yourself is your essence.”