"And therefore, I said, Glaucon, musical training is a more potent instrument than any other, because rhythm and harmony find their way into the inward places of the soul, on which they mightily fasten, imparting grace, and making the soul of him who is rightly educated graceful, or of him who is ill-educated ungraceful..."
--Plato, Republic III.401
"Thus then we reject the professional instruments and also the professional mode of education in music, for in this the performer practices the art, not for the sake of his own improvement, but in order to give pleasure, and that of a vulgar sort, to his hearers. For this reason the execution of such music is not the part of a freeman but of a paid performer, and the result is that the performers are vulgarized, for the end at which they aim is bad. The vulgarity of the spectator tends to lower the character of the music and therefore of the performers; they look to him- he makes them what they are, and fashions even their bodies by the movements which he expects them to exhibit."
--Aristotle, Politics VIII.6