3 ideas
Stare at a wall marked with stains, mixed stones, or clouds, and landscapes, battles, strange faces, and figures in motion will start to appear. Confused, formless material is useful because the mind projects order onto it. That gives the painter compositions they could not have invented directly, which they then refine into proper form.
Air thickens between the eye and a distant object. Far objects therefore look bluer, paler, and less sharply outlined than near ones. The painter should show depth by fading color strength and softening boundaries in proportion to distance, not by relying on linear convergence alone.
A figure's gestures and facial muscles must express its inner state, such as anger, grief, or a man shouting. Without this, the figure is 'twice dead': once because it is only a painting, and again because it shows no motion of the mind. Painters should study these expressions by watching real people in the grip of real emotion and sketching them quickly.