as possible. If we wish to determine that the two colors are physically alike, we seek out the most favorable method for reducing the psychological sources of variation to a minimum. In the spectral photometer the principle of contrast is employed in quite a complicated and unmeasurable way, but the maximum of sensitiveness is obtained and the variations due to psychological influences are quite negligible. On the other hand, if we desire to determine some mental characteristic in regard to the two colors we must obtain sources of light under such conditions that the measurements of their values are carried out with an accuracy to a degree beyond our own sensitiveness; we must not use complicated and unmeasurable psychological arrangements but the simplest ones possible. Under such conditions the variations measured will be due to psychological influences. In both physical and psychological experiments the same fundamental principles are used. The difference lies in the sources of variation; in physics we must eliminate psychological influences, in psychology we must make the physical variations comparatively negligible.
Both physical and psychological measurements are concerned directly with the phenomena of immediate experience. In physics we measure certain objective phenomena of consciousness on one another; in psychology we also measure phenomena of consciousness on one another.[1]
It may seem strange that we should treat the objective phenomena of consciousness as physical phenomena. We are accustomed to think of the physical world as something with a separate existence apart from consciousness. By deductions from the objective phenomena of immediate experience physics has built up a system of independent processes subject to the laws of the conservation of matter and of energy, and expressed in the terms of touch and muscular sensations.[2] The attempt is made to reduce the other phenomena of immediate experience (light, heat, etc.) to these terms, or, as the physicist says, to reduce all physical phenomena to the laws of mechanics. This
