In his Politics (Book I Ch. 5) he claims that some human beings are slaves by nature. Rationality is distinctive of all human beings, but natural slaves possess it in a lesser degree, and so likewise do women, since their distinctive functions also are different. The proper function of women is to obey men. And the proper function of natural slaves is to obey those who are by nature masters, since the former possess sufficient reason to understand rational principles, but not to formulate them for themselves. For Aristotle, then, the fully human life can be lived only by the free-born male citizen.