Aristotle, Nicomachaean Ethics, 9, 1170
“It follows, then, that the most happy person will need to have friends, if he indeed chooses to gaze upon good and appropriate actions. This is what the activities of a good friend are. People also believe that a happy person’s life should be enjoyable. Life is hard for a solitary person. It is not easy to work constantly on your own. It is easier to do with with others and in partnership with them. Activity which is shared—already necessarily pleasant on its own—will be continuous, which is best for a happy person.
But since the perception of the fact that one is good is desirable and this perception is pleasing on its own, we need to share our friend’s perception that they exist. We achieve this by living together and sharing our words and thoughts. This is exactly what someone might call “living together” for human beings: it does not mean just grazing together like cattle.”
ὁ μακάριος δὴ φίλων τοιούτων δεήσεται, εἴπερ θεωρεῖν προαιρεῖται πράξεις ἐπιεικεῖς καὶ οἰκείας· τοιαῦται δ᾿ αἱ τοῦ ἀγαθοῦ φίλου ὄντος.—οἴονταί τε δεῖν ἡδέως ζῆν τὸν εὐδαίμονα. μονώτῃ μὲν οὖν χαλεπὸς ὁ βίος· οὐ γὰρ ῥᾴδιον καθ᾿ αὑτὸν ἐνεργεῖν συνεχῶς, μεθ᾿ ἑτέρων δὲ καὶ πρὸς ἄλλους ῥᾷον. ἔσται οὖν ἡ ἐνέργεια συνεχεστέρα, ἡδεῖα οὖσα καθ᾿ αὑτήν, ὃ δεῖ περὶ τὸν μακάριον εἶναι.
τὸ δ᾿ εἶναι ἦν αἱρετὸν διὰ τὸ αἰσθάνεσθαι αὑτοῦ ἀγαθοῦ ὄντος, ἡ δὲ τοιαύτη αἴσθησις ἡδεῖα καθ᾿ ἑαυτήν· συναισθάνεσθαι ἄρα δεῖ καὶ τοῦ φίλου ὅτι ἔστιν, τοῦτο δὲ γίνοιτ᾿ ἂν ἐν τῷ συζῆν καὶ κοινωνεῖν λόγων καὶ διανοίας· οὕτω γὰρ ἂν δόξειε τὸ συζῆν ἐπὶ τῶν ἀνθρώπων λέγεσθαι, καὶ οὐχ ὥσπερ ἐπὶ τῶν βοσκημάτων τὸ ἐν τῷ αὐτῷ νέμεσθαι.