What enables painting is the perception and representation of intensity. Every point in space has different intensity, which can be represented in painting by black and white and all the gray shades between. In practice, painters can articulate shapes by juxtaposing surfaces of different intensity; by using just color (of the same intensity) one can only represent symbolic shapes. Thus, the basic means of painting are distinct from ideological means, such as geometrical figures, various points of view and organization (perspective), and symbols. For example, a painter perceives that a particular white wall has different intensity at each point, due to shades and reflections from nearby objects, but ideally, a white wall is still a white wall in pitch darkness. In technical drawing, thickness of line is also ideal, demarcating ideal outlines of an object within a perceptual frame different from the one used by painters.

Source: Wikipedia