Plato, Letters 343a

ἢ τὸ ὂν ἑκάστου διὰ τὸ τῶν λόγων ἀσθενές· ὧν ἕνεκα νοῦν ἔχων οὐδεὶς τολμήσει ποτὲ εἰς αὐτὸ τιθέναι τὰ νενοημένα ὑπʼ αὐτοῦ, καὶ ταῦτα εἰς ἀμετακίνητον, ὃ δὴ πάσχει τὰ γεγραμμένα τύποις. τοῦτο δὲ πάλιν αὖ τὸ νῦν λεγόμενον δεῖ μαθεῖν. κύκλος ἕκαστος τῶν ἐν ταῖς πράξεσι γραφομένων ἢ καὶ τορνευθέντων μεστὸς τοῦ ἐναντίου ἐστὶν τῷ πέμπτῳ—τοῦ γὰρ εὐθέος ἐφάπτεται πάντῃ—αὐτὸς δέ, φαμέν, ὁ κύκλος οὔτε τι σμικρότερον οὔτε μεῖζον τῆς ἐναντίας ἔχει ἐν αὑτῷ φύσεως. ὄνομά τε αὐτῶν φαμεν οὐδὲν οὐδενὶ

and for this reason, no man of intelligence will ever venture to commit to it the concepts of his reason, especially when it is unalterable—as is the case with what is formulated in writing. But here again you must learn further the meaning of this last statement. Every one of the circles which are drawn in geometric exercises or are turned by the lathe is full of what is opposite to the fifth, since it is in contact with the straight everywhere ; whereas the circle itself, as we affirm, contains within itself no share greater or less of the opposite nature. And none of the objects, we affirm, has any fixed name,