Plato, Laws 719c

τάδε· παλαιὸς μῦθος, ὦ νομοθέτα, ὑπό τε αὐτῶν ἡμῶν ἀεὶ λεγόμενός ἐστιν καὶ τοῖς ἄλλοις πᾶσιν συνδεδογμένος, ὅτι ποιητής, ὁπόταν ἐν τῷ τρίποδι τῆς Μούσης καθίζηται, τότε οὐκ ἔμφρων ἐστίν, οἷον δὲ κρήνη τις τὸ ἐπιὸν ῥεῖν ἑτοίμως ἐᾷ, καὶ τῆς τέχνης οὔσης μιμήσεως ἀναγκάζεται, ἐναντίως ἀλλήλοις ἀνθρώπους ποιῶν διατιθεμένους, ἐναντία λέγειν αὑτῷ πολλάκις, οἶδεν δὲ οὔτʼ εἰ ταῦτα

These:—There is, O lawgiver, an ancient saying—constantly repeated by ourselves and endorsed by everyone else—that whenever a poet is seated on the Muses’ tripod, he is not in his senses, but resembles a fountain, which gives free course to the upward rush of water and, since his art consists in imitation, he is compelled often to contradict himself, when he creates characters of contradictory moods; and he knows not which of these contradictory utterances is true. But it is not possible for the lawgiver in his law