Aristotle, Nicomachean Ethics
H. Rackham, Ed.

But there is much difference of opinion as to the nature of friendship. Some define it as a matter of similarity; they say that we love those who are like ourselves: whence the proverbs ‘Like finds his like,’ ‘Birds of a feather flock together,’¹ and so on. Others on the contrary say that with men who are alike it is always a case of ‘two of a trade.’² Some try to find a more profound and scientific explanation of the nature of affection. Euripides³ writes that ‘Earth yearneth for the rain’ when dried up, ‘And the majestic Heaven when filled with rain Yearneth to fall to Earth.’ Heracleitus says, ‘Opposition unites,’ and ‘The fairest harmony springs from difference,’ and ‘'Tis strife that makes the world go on.’ Others maintain the opposite view, notably Empedocles, who declares that ‘Like seeks after like.’