"Art and Visual Perception - A Psychology of the Creative Eye"
Rudolf Arnheim
"In van Gogh's painting of his bedroom there are two chairs of approximately identical color, shape, and position. In this way the difference in size, which creates depth, is made much more compelling than it would be if the two objects differed also in shape and color, because it is by similarity that the eye is induced to relate and compare them and thus to discover their difference." P.75