The female, found to be inferior in a moral and political sense, is also considered by Aristotle to be physically inferior to the male, and in the treatise On Generation he deals with this question frequently and at some length:
Male and female differ in their essence by each having a separate ability or faculty, and anatomically by certain parts; essentially the male is that which is able to generate in another, the female is that which is able to generate in itself and out of which comes into being the offspring previously existing in the parent.[10]
The distinction of sex is a first principle:
An animal is not male or female in virtue of an isolated part or an isolated faculty: when that which distinguishes male and female suffers change many other changes accompany it, as would be the case if a first principle is changed.[11]
The treatise is concerned chiefly with the phenomena of reproduction:
For the business of most animals is, you may say, nothing else than to produce young, as the business of a plant is to produce seed and fruit.[12]
Sex-characteristics accordingly are described mainly in accordance with their reproductive functions.
As regards the origin of sex and the causes of male and female, Aristotle is a curious mixture of prejudice and insight. He begins thus:
To suppose that heat and cold are the causes of male and female, or that the different sexes come from the right and left, is not altogether unreasonable in itself, for the right of the body is hotter than the left.