  Experimental psychology had just been founded. Mach’s elements are, quite simply, the variables of a major branch of psychology, the new field of psychophysics. Mach was part of the first generation of physiological psychologists that thought they had broken through the primeval walls dividing the physical from the mental. They thought they had solved this ancient problem through a scientific, monistic unification. The physical was characterized by its ability to be quantified: it was part of science. Now, the realm of Geist was falling under the same methods used so successfully in physics. Following the pioneering work of Fechner, they had measured sensation and found that the relationship of external stimulus to inner response followed mathematical law. Fueled further by developments in evolutionary theory, their optimism soared, perhaps a bit too high. As so often happens in periods of scientific optimism, these new discoveries called for a philosophy in which they would be at home. Mach not only took up this challenge, but as a physicist also applied the results of this tradition to the categories of physics.