Plato, Laws 670b

δεῖ βέλτιον τοὺς πεντηκοντούτας ὅσοισπερ ἂν ᾄδειν προσήκῃ. τῶν γὰρ ῥυθμῶν καὶ τῶν ἁρμονιῶν ἀναγκαῖον αὐτοῖς ἐστιν εὐαισθήτως ἔχειν καὶ γιγνώσκειν· ἢ πῶς τις τὴν ὀρθότητα γνώσεται τῶν μελῶν, ᾧ προσῆκεν ἢ μὴ προσῆκεν τοῦ δωριστί, καὶ τοῦ ῥυθμοῦ ὃν ὁ ποιητὴς αὐτῷ προσῆψεν, ὀρθῶς ἢ μή; δῆλον ὡς οὐδαμῶς. γελοῖος γὰρ ὅ γε πολὺς ὄχλος ἡγούμενος ἱκανῶς γιγνώσκειν τό τε εὐάρμοστον καὶ εὔρυθμον καὶ μή, ὅσοι προσᾴδειν αὐτῶν καὶ βαίνειν ἐν ῥυθμῷ γεγόνασι διηναγκασμένοι,

For they must of necessity possess knowledge and a quick perception of rhythms and harmonies; else how shall a man know which tunes are correct? Obviously he cannot know this at all. It is absurd of the general crowd to imagine that they can fully understand what is harmonious and rhythmical, or the reverse, when they have been drilled to sing to the flute or step