Why is it that one animal is male and another female? This question, addressed by Aristotle throughout his treatise On the Generation of Animals, does not mean merely to inquire into the process by which a particular animal is determined to be one sex rather than another; that question, to which he turns late in his treatise, presupposes the larger and more radical question of why animals should be sexed in the first place. Aristotle provides the broad outline of an answer to this question early in his treatise: in animals that are sexually dimorphic, the male and the female are, he says, the archai geneseōs – the principles of generation, and specifically of animal reproduction, that is, generation of animals of the same kind.

Animal reproduction, the generation by animals of others of the same kind – anthrōpos anthrōpon gennōn, for example, to paraphrase slightly Aristotle's description in the Metaphysics: human begetting human (Metaph. Ζ.7.1032a25) – is a good thing for animals, given that living is a good thing but that animals are mortal.