Plato, Laws 837a

ἐπιθυμίας ἅμα καὶ τῶν λεγομένων ἐρώτων φύσιν ἰδεῖν ἀναγκαῖον, εἰ μέλλει τις ταῦτα ὀρθῶς διανοηθήσεσθαι· δύο γὰρ ὄντα αὐτά, καὶ ἐξ ἀμφοῖν τρίτον ἄλλο εἶδος, ἓν ὄνομα περιλαβὸν πᾶσαν ἀπορίαν καὶ σκότον ἀπεργάζεται. πῶς; φίλον μέν που καλοῦμεν ὅμοιον ὁμοίῳ κατʼ ἀρετὴν καὶ ἴσον ἴσῳ, φίλον δʼ αὖ καὶ τὸ δεόμενον τοῦ πεπλουτηκότος, ἐναντίον ὂν τῷ γένει· ὅταν δὲ ἑκάτερον γίγνηται σφοδρόν, ἔρωτα ἐπονομάζομεν.

and desire and love (so-called), if we are to determine them rightly; for what causes the utmost confusion and obscurity is the fact that this single term embraces these two things, and also a third kind compounded of them both. How so? Friendship is the name we give to the affection of like for like, in point of goodness, and of equal for equal; and also to that of the needy for the rich, which is of the opposite kind; and when either of these feelings is intense we call it love.