CONTRAST, a term used in psychological discussion to designate the tendency of a given mental phenomenon to arouse or to encourage its opposite. Contrast has been alleged as one basis of "idea association," but its most definite demonstrations are found in the domain of sensation, particularly vision. The action may be either "simultaneous" or "successive," according as the aroused opposite appears concomitantly in a separate sensory area, or in the same area as the primary phenomenon at a later time. Thus, a bright surrounding field makes a grey spot look darker, while a dark outlying field causes it to appear brighter, than normal. After stimulation of the retina with white .a dark after-image appears. Colour or chromatic contrast consists of an analogous tendency for each hue to arouse its complementary. Contrast is probably less potent for all aspects of mental life than is the opposed principle of assimilation.