A painting is above all a product of the artist's imagination, it must never be a copy. If, at a later stage, he wants to add two or three touches from nature, of course it doesn't spoil anything.
It is very good to copy what one sees; it is much better to draw what you can't see any more but is in your memory. It is a transformation in which imagination and memory work together. You only reproduce what struck you, that is to say the necessary.