 Thus the great gulf between physical and psychological research persists only when we acquiesce in our habitual stereotyped conceptions. A color is a physical object as soon as we consider its dependence, for instance, upon its luminous source, upon other colors, upon temperatures, upon spaces, and so forth. When we consider, however, its dependence upon the retina…it is a psychological object, a sensation. Not the subject matter, but the direction of investigation, is different in the two domains. (Mach 1886: 17–18)