You should choose an old tumbledown wall and throw over it a piece of
white silk. Then morning and evening you should gaze at it, until at
length you can see the ruin through the silk--its prominences, its
levels, its zigzags, and its cleavages, storing them up in the mind and
fixing them in the eye. Make the prominences your mountains, the lower
parts your water, the hollows your ravines, the cracks your streams, the
lighter parts your nearer points, the darker parts your more distant
points. Get all these thoroughly into you, and soon you will see men,
birds, plants, and trees, flying and moving among them. You may then ply
your brush according to your fancy, and the result will be of heaven,
not of men.
_Sung Ti_ (Chinese, eleventh century).