While a Roman nobleman was one day endeavoring to drive a hard bargain
with Salvator Rosa, he coolly interrupted him, saying that, till the
picture was finished, he himself did not know its value; "I never
bargain, sir, with my pencil; for it knows not the value of its own
labor before the work is finished. When the picture is done, I will let
you know what it costs, and you may then take it or not as you please."