Nicomachean Ethics

Yet the disputed points respecting it are not few: some men lay down that it is a kind of resemblance, and that men who are like one another are friends: whence come the common sayings, “Like will to like,” “Birds of a feather,” and so on. Others, on the contrary, say, that all such come under the maxim, “Two of a trade never agree.”[2]

Again, some men push their enquiries on these points higher and reason physically: as Euripides, who says,

“The earth by drought consumed doth love the rain, And the great heaven, overcharged with rain, Doth love to fall in showers upon the earth.”

Heraclitus, again, maintains, that “contrariety is expedient, and that the best agreement arises from things differing, and that all things come into being in the way of the principle of antagonism.”

Empedocles, among others, in direct opposition to these, affirms, that “like aims at like.”

These physical questions we will take leave to omit, inasmuch as they are foreign to the present enquiry; and we will examine such as are proper to man and concern moral characters and feelings: as, for instance, “Does Friendship arise among all without distinction, or is it impossible for bad men to be friends?” and, “Is there but one species of Friendship, or several?” for they who ground the opinion that there is but one on the fact that Friendship admits of degrees hold that upon insufficient proof; because things which are different in species admit likewise of degrees (on this point we have spoken before).