In all the liberal and ingenious arts, in painting, in poetry, in music, in eloquence, in philosophy, the great artist feels always the real imperfection of his own best works, and is more sensible than any man how much they fall short of that ideal perfection of which he has formed some conception, which he imitates as well as he can, but which he despairs of ever equalling.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“His whole mind, in short, is deeply impressed, his whole behaviour and deportment are distinctly stamped with the character of real modesty; with that of a very moderate estimation of his own merit, and, at the same time, of a full sense of the merit of other people. In all the liberal and ingenious arts, in painting, in poetry, in music, in eloquence, in philosophy, the great artist feels always the real imperfection of his own best works, and is more sensible than any man how much they fall short of that ideal perfection of which he has formed some conception, which he imitates as well as he can, but which he despairs of ever equalling. It is the inferior artist only, who is ever perfectly satisfied with his own performances. He has little conception of this ideal perfection, about which he has little employed his thoughts; and it is chiefly to the works of other artists, of, perhaps, a still lower order, that he deigns to compare his own works.” source