GUSTAV THEODOR FECHNER. — German philosopher, b. Gross-Särchen (Lower Lusatia) on 19 Apr. 1801, d. after a long university career in Leipzig on 16 Nov. 1887. In his thought, strongly imbued with naturalistic ideas and theosophy, various principles converge from materialism to idealism: however, he represented a reaction to the a priori method of the Kantian and Hegelian schools.
In *Nanna oder das Seelenleben der Pflanzen* (Leipzig 1848) he defended universal cosmic animism, which in *Zend-Avesta* (ibid. 1851) took on a Hermetic and theosophical tone. His fame is linked to the *Elemente der Psychophysik* (ibid. 1860), in which he expounded the well-known psychological law that bears his name, according to which “the intensity of sensation is equal to the logarithm of the corresponding stimulus” (S = C log. R): that is, for sensation to increase in arithmetic progression, the stimulus must increase in geometric progression. With this law, F. gave decisive impetus to modern experimental psychology.