Naturalistic imitation is no longer the guiding principle of painting. We have set our faces toward the abstract, the symbolic, the subjectively conceived. The modernist, with an abstract conception, cares less for the illusion of bulk, in many cases, than for mobility of line. This is no new phenomenon in the history of art. Painting has always been conceived primarily in three dimensions or primarily in two according to the degree of concreteness that the painters have sought.
Line is perhaps the most direct vehicle of thought and feeling.
A rendering in two dimensions tends to be general. Line can suggest a third dimension, but cannot show it. When form is modeled up in light and shade, the rendering becomes correspondingly more specific, more concrete. Symbols turn into facts, and tend to be seen with the eyes instead of with the mind. The tendencies of modern architecture, too, contribute to the growing popularity of these media.