Page "Genus–differentia definition" Paragraph 29

This can be clarified with a well-known example.

Suppose we wanted to define the phrase human being.

Following the ancient Greeks ( Socrates and his successors ) and modern biologists, we say that human being is a species and that each individual person is a member of the species human being.

So we ask what the genus, or general category, of the species is ; the Greeks ( but not the biologists ) would say that the genus is animal.

What is the differentia of the species, that is, the distinguishing characteristic of human being that other animals do not have?

The Greeks said it is rationality ; thus, Aristotle said, A human being is a rational animal.