Fechner Gustav Theodor (1801–87), German physicist and philosopher whose Elemente der Psychophysik (1860; English translation, 1966) inaugurated experimental psychology. Obsessed with the mind–body problem, Fechner advanced an identity theory in which every object is both mental and physical, and in support invented psychophysics – the ‘exact science of the functional relation. . . between mind and body.’ Fechner began with the concept of the limen, or sensory threshold. The absolute threshold is the stimulus strength (R, Reiz) needed to create a conscious sensation (S), and the relative threshold is the strength that must be added to a stimulus for a just noticeable difference (jnd) to be perceived. E. H. Weber (1795–1878) had shown that a constant ratio held between relative threshold and stimulus magnitude, Weber’s law: DR/R % k. By experimentally determining jnd’s for pairs of stimulus magnitudes (such as weights), Fechner formulated his ‘functional relation,’ S % k log R, Fechner’s law, an identity equation of mind and matter. Later psychophysicists replaced it with a power law, R % kSn, where n depends on the kind of stimulus. The importance of psychophysics to psychology consisted in its showing that quantification of experience was possible, and its providing a general paradigm for psychological experimentation in which controlled stimulus conditions are systematically varied and effects observed. In his later years, Fechner brought the experimental method to bear on aesthetics (Vorschule der Aesthetik, 1876). T.H.L.