good art means the ability of any one man to pin down in some permanent and intelligible medium a sort of idea of what he sees in Nature that nobody else sees. In other words, to make the other fellow grasp, through skilled selective care in interpretative reproduction or symbolism, some inkling of what only the artist himself could possibly see in the actual objective scene itself.
Letter to Woodburn Harris (25 February-1 March 1929), quoted in "H.P. Lovecraft, a Life" by S.T. Joshi, p. 487