Plato, Letters 342a

σέμνʼ ἄττα μεμαθηκότας. ἔτι δὲ μακρότερα περὶ αὐτῶν ἐν νῷ μοι γέγονεν εἰπεῖν· τάχα γὰρ ἂν περὶ ὧν λέγω σαφέστερον ἂν εἴη λεχθέντων αὐτῶν. ἔστι γάρ τις λόγος ἀληθής, ἐναντίος τῷ τολμήσαντι γράφειν τῶν τοιούτων καὶ ὁτιοῦν, πολλάκις μὲν ὑπʼ ἐμοῦ καὶ πρόσθεν ῥηθείς, ἔοικεν δʼ οὖν εἶναι καὶ νῦν λεκτέος.ἔστιν τῶν ὄντων ἑκάστῳ, διʼ ὧν τὴν ἐπιστήμην ἀνάγκη παραγίγνεσθαι, τρία, τέταρτον δʼ αὐτή—πέμπτον δʼ αὐτὸ

But concerning these studies I am minded to speak still more at length; since the subject with which I am dealing will perhaps be clearer when I have thus spoken. For there is a certain true argument which confronts the man who ventures to write anything at all of these matters, an argument which, although I have frequently stated it in the past, seems to require statement also at the present time. Every existing object has three things which are the necessary means by which knowledge of that object is acquired; and the knowledge itself is a fourth thing; and as a fifth one must postulate the object itself which is cognizable