The Republic - pg. 80

"So, Glaucon," I said, "isn't this why the rearing in music
is most sovereign? Because rhythm and harmony most of
all insinuate themselves into the innermost part of the
soul and most vigorously lay hold of it in bringing grace
with them; and they make a man graceful if he is correctly
reared, if not, the opposite. Furthermore, it is sovereign
because the man properly reared on rhythm and harmony
would have the sharpest sense for what's been left out and
what isn't a fine product of craft or what isn't a fine product
of nature.