PAINTING.

The mind of the painter must resemble a mirror, which always takes
the colour of the object it reflects and is completely occupied by
the images of as many objects as are in front of it. Therefore you
must know, Oh Painter! that you cannot be a good one if you are not
the universal master of representing by your art every kind of form
produced by nature. And this you will not know how to do if you do
not see them, and retain them in your mind. Hence as you go through
the fields, turn your attention to various objects, and, in turn
look now at this thing and now at that, collecting a store of divers
facts selected and chosen from those of less value. But do not do
like some painters who, when they are wearied with exercising their
fancy dismiss their work from their thoughts and take exercise in
walking for relaxation, but still keep fatigue in their mind which,
though they see various objects [around them], does not apprehend
them; but, even when they meet friends or relations and are saluted
by them, although they see and hear them, take no more cognisance of
them than if they had met so much empty air.

Useful games and exercises (507. 508).