Let us agree as to the meaning of the word "finished." What finishes a
picture is not the quantity of detail in it, but the rightness of the
general effect. A picture is not limited only by its frame. Whatever be
the subject, there must be a principal object on which your eyes rest
continually: the other objects are only the complement of this, they are
less interesting to you; and after that there is nothing more for your
eye.
There is the real limit of your picture. This principal object must seem
so to the spectator of your work. Therefore, one must always return to
this, and state its colour with more and more decision.
_Rousseau._