Abstract Gustav Theodor Fechner (b. 1801–d. 1887) is well known to psychologists as the founder of psychophysics, a set of methods for empirically relating measured sensory stimulus to reported sensation. Rich as this field of research has proven to be, especially as detection instruments and recording devices have improved in modern times, Fechner himself would be disappointed to discover that he is remembered merely for psychophysics as psychologists understand it today. To Fechner, those particular methods and approaches were only one part of the domain of psychophysics—outer psychophysics—whereas he envisioned psychophysics (both outer and inner) to be the key to the broadest kind of scientific worldview: the study of the relationship between the material world and the mental world (indeed the spiritual universe; in German, die geistige Welt). The preface of Fechner’s 1860 masterwork, Elements of Psychophysics, states, “[I]t is an exact theory of the relation of mind to body. . . . As an exact science psychophysics, like physics, must rest on experience and the mathematical connection of those empirical facts that demand a measure of what is experienced or, when such a measure is not available, a search for it.” In that definition, Fechner reveals his view of natural science, and how psychophysics was meant to expand the range of natural science and the scope of its achievements. Fechner’s influence emerged during the mid-19th century, as the physical and medical sciences were achieving great breakthroughs and establishing fundamental and unifying concepts; this was especially true in Germany, including the city where he came to study and remained for the rest of his life, Leipzig. Fechner was one of the most enthusiastic and optimistic believers in unifying concepts of science. Fechner’s psychophysics gave important impetus to psychometrics and experimental psychology; he also proposed a statistical approach to aesthetics and a “theory of collectives” that pointed toward statistical interpretations of many (perhaps all) areas of experience. In his early career he was a central figure in German physics and an early supporter of the atomic theory of matter as well as Darwinian evolution. His profoundly spiritual ideas, however, were out of step with the science of his time. This aspect of Fechner’s thought is evident in his inner psychophysics, where he searched for the full measure of “what is experienced.”