W E LIVE AND ACT in a world which we perceive as relatively stable in spite of the ever-changing impingements on our sense organs. This fact becomes a problem to those interested in evolving simplified conceptual explanatory systems. It has interested philosophers of all times and has troubled psychologists ever since that discipline was born. It has helped to form the foundation of one psychological system and to work the downfall of another. Whatever may be the ultimate evaluation of psychological theorists, on one statement all can agree: it is this simple fact of perceptual constancy that makes effective actions possible, from the simplest to the most complex, from walking across