A valuable excerpt from Plato’s REPUBLIC

But it does not require much analysis to see that grace or the absence of grace accompanies good or bad rhythm.
None at all.
And also that good and bad rhythm naturally assimilate to a good and bad style; and that harmony and discord in like manner follow style; for our principle is that rhythm and harmony are regulated by the words, and not the words by them.
Just so, he said, they should follow the words.
And will not the words and the character of the style depend on the temper of the soul?
And everything else on the style?
Then beauty of style and harmony and grace and good rhythm depend on simplicity, – I mean the true simplicity of a rightly and nobly ordered mind and character, not that other simplicity which is only a euphemism for folly?
Very true, he replied.

[Socrates expands on the role of the artist in the ideal State and argues that unsuitable artists should be prevented from practising their art.]

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: and also because he who has received this true education of the inner being will most shrewdly perceive omissions or faults in art and nature, and with a true taste, while he praises and rejoices over and receives into his soul the good, and becomes noble and good, he will justify blame and hate the bad, now in the days of his youth, even before he will recognize and salute the friend with whom his education has made him long familiar.
Yes, he said, I quite agree with you in thinking that it is for such reasons that they should be trained in music……….
Even so, as I maintain, neither we nor the guardians, whom we say that we have to educate, can ever become musical until we and they know the essential forms of temperance, courage, liberality, magnanimity, and their kindred, as well as the contrary forms, in all their combinations, and can recognise then and their images wherever they are found, not slighting them either in small things or great, but believing them all to be within the sphere of one art and study.
Most assuredly.
And then nobility of soul is observed in harmonious union with beauty of form, and both are cast from the same mould, that will be the fairest of sights to him who has en eye to see it?
The fairest indeed.
And the fairest is also the loveliest?
That may be assumed.
And it is with human beings who most display such harmony that a musical man will be most in love; but he will not love any who do not possess it.
That is true, he replied, if the deficiency be in the soul; but if there be any bodily defect he will be patient of it, and may even approve it.

[A short discussion of the nature of pleasure.]

Thus much of music, and the ending is appropriate; for what should be the end of music if not the love of beauty?