Plato on Music

[...] they involuntarily, through their ignorance, asserted falsely that music did not possess any correctness whatever; but that it might be judged of most correctly by the pleasure of the party gratified, whether he were a better person or a worse.

Plato, The Laws, bk. 3, ch. 15.
(Plato, The Works of Plato: A New and Literal Version, trans. George Burges, London: Henry G. Bohn, 1852, vol. 5, The Laws, p. 117.)

without intending it, they were guilty of so far slandering their art as to assert, in their folly, that there was no such thing as right or wrong in music: the one proper criterion was the pleasure of the hearer, be he gentle or simple.

Plato, The Laws, bk. 3.
(Plato, The Laws of Plato, ed. w. intro., notes, etc. by E. B. England, Manchester: The University Press; London, New York, Bombay, etc.: Longmans, Green & Co., 1921, vol. 1, bks. 1-6, pp. 114, 409.)