Plato, Letters 342b

τιθέναι δεῖ ὃ δὴ γνωστόν τε καὶ ἀληθῶς ἐστιν ὄν—ἓν μὲν ὄνομα, δεύτερον δὲ λόγος, τὸ δὲ τρίτον εἴδωλον, τέταρτον δὲ ἐπιστήμη. περὶ ἓν οὖν λαβὲ βουλόμενος μαθεῖν τὸ νῦν λεγόμενον, καὶ πάντων οὕτω πέρι νόησον. κύκλος ἐστίν τι λεγόμενον, ᾧ τοῦτʼ αὐτό ἐστιν ὄνομα ὃ νῦν ἐφθέγμεθα. λόγος δʼ αὐτοῦ τὸ δεύτερον, ἐξ ὀνομάτων καὶ ῥημάτων συγκείμενος· τὸ γὰρ ἐκ τῶν ἐσχάτων ἐπὶ τὸ μέσον ἴσον ἀπέχον πάντῃ, λόγος ἂν εἴη ἐκείνου ᾧπερ στρογγύλον καὶ περιφερὲς

and true. First of these comes the name; secondly the definition; thirdly the image; fourthly the knowledge. If you wish, then, to understand what I am now saying, take a single example and learn from it what applies to all. There is an object called a circle, which has for its name the word we have just mentioned and, secondly, it has a definition, composed of names and verbs; for that which is everywhere equidistant from the extremities to the center will be the definition of that object which has for its name round and spherical and circle.