The Power of Music

And therefore, I [Socrates] 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, The Republic of Plato, 3rd ed., tr. Benjamin Jowett, Oxford: Clarendon Press, 1888, p. 88.