Discourses: Book 4 11 : 1
Some people raise the question whether the social instinct is a necessary element in the nature of man; nevertheless, even these people, as it seems to me, would not question that the instinct of cleanliness is most assuredly a necessary element, and that man is distinguished from the animals by this quality if by anything. When, therefore, we see some other animal cleaning itself, we are in the habit of saying in surprise that it is acting "like a human being." And again, if one finds fault with some beast, we are in the habit of saying immediately, as though in apology, "Well, of course it isn't a human being." So true it is that we consider cleanliness to be a special characteristic of man, deriving it in the first instance from the gods. For since they are by nature pure and undefiled, in so far as men have approached them by virtue of reason, just so far are they attached to purity and cleanliness. But since it is impossible for the nature of men to be altogether pure, seeing that it is composed of such material as it is, the reason which they have received from the gods endeavours to render this material clean as far as is possible.