If you paint the leaf on a tree without using a model, your imagination will only supply you with a few leaves; but Nature offers you millions, all on the same tree. No two leaves are exactly the same. The artist who paints only what is in his mind must very soon repeat himself.
Pierre-Auguste RenoirWhen I've painted a woman's bottom so that I want to touch it, then [the painting] is finished.
Pierre-Auguste Renoir