On April 19, 1801, German philosopher, physicist and experimental psychologist Gustav Theodor Fechner was born. An early pioneer in experimental psychology and founder of psychophysics, he inspired many 20th century scientists and philosophers. He is also credited with demonstrating the non-linear relationship between psychological sensation and the physical intensity of a stimulus, which became known as the Weber–Fechner law.
Gustav Fechner was educated in Sorau and studied medicine at the Medizinisch-Chirurgische Akademie in Dresden starting from 1817, but later switched to Leipzig. In 1834, he was appointed professor of physics, however, after contracting an eye disorder, he turned to the study of the mind and its relations with the body, giving public lectures on the subjects dealt with in his books.
Fechner also became increasingly enthusiastic about philosophy and probably the ultimate philosophic problem which concerned him, and to which his psychophysics was a solution, was the perennial mind-body problem. His solution has been called the identity hypothesis. It means that mind and body are not regarded as a real dualism, but are different sides of one reality. They are separated in the form of sensation and stimulus. That is, what appears from a subjective viewpoint as the mind, but it appears from an external or objective viewpoint as the body. In Fechner’s law (sensation intensity = C log stimulus intensity), it becomes evident that the dualism is not real. While this law has been criticized as illogical, and for not having universal applicability, it has been useful in research on hearing and vision.
Gustav Fechner’s most influential cotribution were probably his ‘Elemente der Psychophysik’, published in 1860 and his ‘Revision der Hauptpunkte der Psychophysik’, published in 1882. Upon these works mainly rests Fechner’s fame as a psychologist. He established new methods of mental measurement, hence, beginning of quantitative experimental psychology. As part of the measurement were the method of just-noticeable differences, the method of constant stimuli, and the method of average error. Along with Wilhelm Wundt and Hermann von Helmholtz, Gustav Fechner is recognized as one of the founders of modern experimental psychology.
At yovisto academic video search, you can learn more about the Fechner-Weber Law in a short lecture.
References and Further Reading:
-  Gustav Fechner Biography
-  Gustav Fechner at britannica
-  Gustav Fechner Biography, Bibliography, and sources
-  Gustav Fechner at Wikidata
-  Gustav Fechner at Reasonator