Discourses, "About Purity Cleanliness" (§1)

“Some persons raise a question whether the social feeling is contained in the nature of man; and yet I think that these same persons would have no doubt that love of purity is certainly contained in it, and that if man is distinguished from other animals by anything, he is distinguished by this. When then we see any other animal cleaning itself, we are accustomed to speak of the act with surprise, and to add that the animal is acting like a man; and on the other hand, if a man blames an animal for being dirty, straightway, as if we were making an excuse for it, we say that of course the animal is not a human creature.”