This extract is from The Republic by Plato, Book VII (522-522c).
Music, he said, as you will remember, was the counterpart of gymnastic, and trained the guardians by the influences of habit, by harmony making them harmonious, by rhythm rhythmical, but not giving them science; and the words, whether fabulous or closer to the truth, were meant to impress upon them habits similar to these. But in music there was nothing which tended to that good which you are now seeking.
You are most accurate, I said, in your reminder; in music there certainly was nothing of the kind. But what branch of knowledge is there, my dear Glaucon, which is of the desired nature; since all the useful arts were reckoned mean by us?
Undoubtedly; and yet what study remains, distinct both from music and gymnastic and from the arts?
Well, I said, if nothing remains outside them, let us select something which is a common factor in all.
What may that be?
Something, for instance, which all arts and sciences and intelligences use in common, and which everyone has to learn among the first elements of education.
What is that?
The little matter of distinguishing one, two, and three – in a word, number and calculation: – do not all arts and sciences make use of them?
Yes.
The Dialogues of Plato, translated by Benjamin Jowett, Volume Four, The Republic, edited by M Hare & DA Russell, Sphere Books Ltd., 1970, Book VII (398-403), pp.304 -305.
Theory of Music 