The beauty of poetry is a matter of such nicety, that a young beginner can scarce ever be certain that he has attained it.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“There are others, in which the success admits, either of clear demonstration, or very satisfactory proof. Among the candidates for excellence in those different arts, the anxiety about the public opinion is always much greater in the former than in the latter. The beauty of poetry is a matter of such nicety, that a young beginner can scarce ever be certain that he has attained it. Nothing delights him so much, therefore, as the favourable judgments of his friends and of the public; and nothing mortifies him so severely as the contrary. The one establishes, the other shakes, the good opinion which he is anxious to entertain concerning his own performances.” source