Plato, Letters 342c

ὄνομα καὶ κύκλος. τρίτον δὲ τὸ ζωγραφούμενόν τε καὶ ἐξαλειφόμενον καὶ τορνευόμενον καὶ ἀπολλύμενον· ὧν αὐτὸς ὁ κύκλος, ὃν πέρι πάντʼ ἐστὶν ταῦτα, οὐδὲν πάσχει, τούτων ὡς ἕτερον ὄν. τέταρτον δὲ ἐπιστήμη καὶ νοῦς ἀληθής τε δόξα περὶ ταῦτʼ ἐστίν· ὡς δὲ ἓν τοῦτο αὖ πᾶν θετέον, οὐκ ἐν φωναῖς οὐδʼ ἐν σωμάτων σχήμασιν ἀλλʼ ἐν ψυχαῖς ἐνόν, ᾧ δῆλον ἕτερόν τε ὂν αὐτοῦ τοῦ κύκλου τῆς φύσεως τῶν

And in the third place there is that object which is in course of being portrayed and obliterated, or of being shaped with a lathe, and falling into decay; but none of these affections is suffered by the circle itself, whereto all these others are related inasmuch as it is distinct therefrom. Fourth comes knowledge and intelligence and true opinion regarding these objects; and these we must assume to form a single whole, which does not exist in vocal utterance or in bodily forms but in souls; whereby it is plain that it differs both from the nature of the circle itself and from the three previously mentioned. And of those four