“A painter should not object to listening to the opinion of a layman.  Even if a man is not a painter, he knows what the human form looks like and whether a man has, let’s say, a hump or a thick leg or a large hand, or whether he is lame or has a large mouth or nose or anything else.  If someone is quite capable of judging the works of Nature, shouldn’t we admit him capable of detecting our errors?

Leonardo Da Vinci’s Advice to Artists – edited by Emery Kelen