1801–1887 · physicist, philosopher, founder of psychophysics

ustav Theodor Fechner trained in medicine at Leipzig, became a professor of physics there, and is now remembered as the founder of psychophysics: the measurement of the relation between physical stimuli and sensations.

a life in three parts

His early career was in physics, including work on electrical currents. Around 1840, after experiments on afterimages that involved looking toward the sun, he suffered damage to his eyes and a long illness, and gave up his chair. He recovered over several years and turned to philosophy, defending the view that mind and matter are two sides of one reality. By his own account, the idea for a mathematical law linking them came to him on the morning of 22 October 1850; students of perception still mark that date, and the International Society for Psychophysics calls its annual meeting Fechner Day.

what he did

In Elemente der Psychophysik (Fechner, 1860) Fechner took Weber's finding about just-noticeable differences and argued that if each JND is one equal step of sensation, then sensation grows with the logarithm of stimulus intensity [paraphrase, not a quotation]. This is Fechner's law. More lasting than the law were the methods he systematized for measuring thresholds: the method of limits, the method of constant stimuli, and the method of adjustment. Every psychophysics lab still uses descendants of all three.

Later in life he founded experimental aesthetics, asking people to choose among rectangles of different proportions. He also wrote satirical pieces under the pen name "Dr. Mises".

what did not hold

The logarithmic law is only an approximation; later work found that many sensations follow power functions better. And the fixed threshold that his methods were built to find was recast by signal detection theory as a decision on noisy evidence (see signal detection and the essay where the line is). His central claim, that the mental can be measured, is the one that stuck.

[playful] The research pages keep a night shift. Type its first word on any of them and the lights go down; type it again for day.

The tour has a psychology hall with portraits on the walls; the frames there belong to the tour's own story. The plain tour lists them.

Primary works and sources

1. Fechner (1860). Elemente der Psychophysik. Leipzig: Breitkopf und Härtel.

Try it yourself. [playful] Small toys on the toy shelf that show this in action:   • the line judge — find your Weber fraction for line length

see also

	essay: where the line is — starts with Fechner's threshold
	method: the staircase — a modern descendant of the method of limits
	timeline: psychophysics
	measurement — what changes when somebody looks