Gustav Fechner

Gustav Theodor Fechner was a German physicist, philosopher, and experimental psychologist who pioneered experimental psychology and founded psychophysics, a set of techniques for measuring the mind. His work inspired many scientists and philosophers in the 20th century. He is known for showing that the relationship between psychological sensation and the physical intensity of a stimulus is non-linear, expressed in the formula S = c log R, which came to be called the Weber–Fechner law.

Fechner was born on 19 April 1801 in Groß Särchen, near Muskau in Lower Lusatia. His father, a maternal uncle, and his paternal grandfather were pastors. His mother, Johanna Dorothea Fechner (born 1774, née Fischer), also came from a religious family. Some biographers believe Fechner became an atheist later in life, while others say he developed his own religious system blending Christianity and paganism. His father, Samuel Traugott Fischer Fechner (1765–1806), was free-thinking: he had his children vaccinated, taught them Latin, and was a passionate fruit grower. He died unexpectedly in 1806, leaving the family destitute. Fechner had an elder brother, Eduard Clemens Fechner (1799–1861), and three younger sisters: Emilie, Clementine, and Mathilde. After their father’s death, Fechner and his brother were raised for a few years by their maternal uncle, a pastor, before rejoining their mother and sisters in Dresden. Fechner first attended school in Sorau (now Żary in western Poland). In 1817, he studied medicine for six months in Dresden, then from 1818 at the University of Leipzig, where he spent the rest of his life. He earned his PhD from Leipzig in 1823.

In 1834, Fechner became a professor of physics at Leipzig. In 1839, while studying color and vision, he injured his eyes by gazing at the Sun through colored glasses to research afterimages. After much suffering, he resigned from his position. During his recovery, he turned to studying the mind and its relation to the body, giving public lectures on topics from his books. While lying in bed, he had an insight into the relationship between mental and material sensations, which proved significant for psychology by establishing a quantitative link between the mental and physical worlds.

Fechner published chemical and physical papers and translated chemical works by Jean-Baptiste Biot and Louis Jacques Thénard from French. He also wrote poems and humorous pieces, such as *Vergleichende Anatomie der Engel* (1825) under the pseudonym "Dr. Mises." His epoch-making work was *Elemente der Psychophysik* (1860). He started from the monistic idea that bodily facts and conscious facts, though not reducible to each other, are different sides of one reality. His originality lay in trying to find an exact mathematical relation between them. The most famous outcome is Fechner’s law: for a sensation to increase in arithmetical progression, the stimulus must increase in geometrical progression. This law has been found useful but fails for very faint and very strong sensations. Within its useful range, sensation is a logarithmic function of physical intensity. S. S. Stevens later pointed out that the law does not account for the fact that perceived relationships among stimuli (e.g., shades of grey) remain unchanged with changes in overall intensity. In his 1961 paper "To Honor Fechner and Repeal His Law," Stevens proposed that intensity of stimulation relates to perception via a power-law. Fechner’s general formula for sensation units is S = c log R, where S is sensation, R is the stimulus numerically estimated, and c is a constant determined by experiment for each order of sensibility. This reasoning has been criticized because, as William James noted, sensations are indivisible units, not masses of units combined.

In 1838, Fechner studied the still-mysterious perceptual illusion now called the Fechner color effect, where colors appear in a moving black-and-white pattern. The English journalist and amateur scientist Charles Benham, in 1894, helped English-speakers learn of the effect through his spinning top, Benham’s top. Whether Fechner and Benham ever met is unknown. In his *Vorschule der Aesthetik* (1876), Fechner used the method of extreme ranks for subjective judgments. Two years later, he published a paper developing the notion of the median and is generally credited with introducing it into formal data analysis. He later explored experimental aesthetics, trying to determine the shapes and dimensions of aesthetically pleasing objects by using the sizes of paintings as a database. In 1871, Fechner reported the first empirical survey of colored letter photisms among 73 synesthetes, a line of work followed in the 1880s by Francis Galton. One of Fechner’s speculations about consciousness dealt with the brain. During his time, it was known that the brain is bilaterally symmetrical and that there is a deep division between the two halves. He died on 18 November 1887.