He postulated that mind and body, though appearing to be separate entities, are actually different sides of one reality. He also developed experimental procedures, still useful in experimental psychology, for measuring sensations in relation to the physical magnitude of stimuli.
Philosophically Fechner defended a monism in which the one world can be seen in one way physically and in the other mentally. Experimentally he sought to confirm this insight by discovering close quantitative relationships between conscious experience and physiological stimulus, eventually discovering the law that the intensity of a sensation increases as the log of the stimulus (S = k log R) characterizing psychophysical relations.
This law proved the existence of evidence-based connection between body and psyche. This formula was named the Fechner-Weber law, because it based on the theory of the just-noticeable difference, advanced earlier by Ernst Heinrich Weber.
He developed experimental procedures for measuring sensations in relation to the physical magnitude of stimuli. He proposed the three methods of measurement were the method of just-noticeable differences, the method of constant stimuli, and the method of average error. According to the authorities, the method of constant stimuli, called also the method of right and wrong cases, has become the most important of the three methods.
In philosophy he was also an animist, maintaining that life is manifest in all objects of the universe.
Fechner called his conception Tagesansicht (Day Vision), unlike his contemporary materialism - Nachtansicht (Night Vision). His greatest achievement was in the investigation of exact relationships in psychology and aesthetics. He demonstrated that since the mind can be measurabled and understood mathematically, it is possible calculation in psychology. He hoped to organize psychophysics and metaphysics in a way that united philosophy and the human sciences. In 1865 Fechner's interest turned to the study of the basic aesthetic principles of art. In 1876 he published his famous Vorschule der Aesthetik .
Fechner’s experimental method became the basis for experimental psychology and later inspired Wilhelm Wundt, who created the first scientific Psychological laboratory.
Sigmund Freud admired Fechner as the pioneer of psychophysics and a founder of scientific and experimental psychology. He attended Fechner's lectures in Leipzig in 1874. He called him "The great G. T. Fechner."
William James, who did not care for quantitative analysis or the statistical approach in psychology, dismisses the psychophysic law as an "idol of the den," the psychological outcome of which is nothing.
Although he was educated in biological and medical science, Fechner turned to mathematics and physics.
In 1834 he was appointed professor of physics at the University of Leipzig, but in 1839 he fell ill with an eye disorder while studying the phenomena of color and experimenting on afterimages by gazing at the sun. After much suffering he resigned and isolated himself from the world for 3 years. Later Fechner described his experiences while ill and the existential crisis and depression that followed.
Subsequently recovering, Fechner developed his interest in sensation and panpsychis, he turned to the study of the mind and its relations with the body.
He authored many books and monographs on such diverse subjects as medicine, esthetics, and experimental psychology.
He started to recover in 1842 and competed this in the mid 40s. In 1851 he was allowed back at Leipzig. In the course of this second creative period, he set out the foundations of psychophysics, such as the Fechner-Weber law, by which he is remembered as a founder of experimental psychology. His two-volume Elemente der Psychophysik was published in 1860. From about 1865 he delved into experimental aesthetics and sought to determine by actual measurements which shapes and dimensions are most aesthetically pleasing.
Gustav Fechner died in 1889 at the age of 88.
Fechner was the brother of portrait painter and an etcher Eduard Clemens Fechner and of Clementine Wieck Fechner, who was the stepmother of Clara Wieck, a famous German musician and composer.
In 1833 Fechner married Clara Maria Volkmann, the sister of his friend Alfred W. Volkmann, a famous physiologist, anatomist, and philosopher, who also worked at the University of Leipzig. Gustav and Clara had no children.