Plato, Euthyphro, Apology, Crito, Phaedo, Φαίδων 408:

ἀλλὰ μὴν ἡμεῖσ γε τὰ ὁρατὰ καὶ τὰ μὴ τῇ τῶν ἀνθρώπων φύσει ἐλέγομεν· ἢ ἄλλῃ τινὶ οἰεί;