Plato, Phaedo 100b (Go to the Scaife viewer for the full text)
“But I am saying nothing new. And I have never stopped saying these things at different times and in previous arguments. And I am really going to try to demonstrate to you the kind of cause which I have been on about. I am going back to those things everyone knows and I am starting from them.
Once I have proposed that that there is a thing called the beautiful on its own and also the good and the big and all the rest of the things, if you grant these to me and concede that they exist, I hope to show the cause from these things and to prove that the soul is immortal.
Look at the things that follow from these assumptions if it occurs to you as it does to me. For it seems to me that if any other thing is beautiful apart from beauty itself it is not because some other single type of beauty exists more than because it has a share of that initial beauty?”
᾿Αλλ’, ἦ δ’ ὅς, ὧδε λέγω, οὐδὲν καινόν, ἀλλ’ ἅπερ ἀεί τε ἄλλοτε καὶ ἐν τῷ παρεληλυθότι λόγῳ οὐδὲν πέπαυμαι λέγων. ἔρχομαι [γὰρ] δὴ ἐπιχειρῶν σοι ἐπιδείξασθαι τῆς αἰτίας τὸ εἶδος ὃ πεπραγμάτευμαι, καὶ εἶμι πάλιν ἐπ’ ἐκεῖνα
τὰ πολυθρύλητα καὶ ἄρχομαι ἀπ’ ἐκείνων, ὑποθέμενος εἶναί τι καλὸν αὐτὸ καθ’ αὑτὸ καὶ ἀγαθὸν καὶ μέγα καὶ τἆλλα πάντα· ἃ εἴ μοι δίδως τε καὶ συγχωρεῖς εἶναι ταῦτα, ἐλπίζω σοι ἐκ τούτων τὴν αἰτίαν ἐπιδείξειν καὶ ἀνευρήσειν ὡς ἀθάνατον [ἡ] ψυχή. Σκόπει δή, ἔφη, τὰ ἑξῆς ἐκείνοις ἐάν σοι συνδοκῇ ὥσπερ ἐμοί. φαίνεται γάρ μοι, εἴ τί ἐστιν ἄλλο καλὸν πλὴν αὐτὸ τὸ καλόν, οὐδὲ δι’ ἓν ἄλλο καλὸν εἶναι ἢ διότι μετέχει ἐκείνου τοῦ καλοῦ·