Gustav Theodor Fechner

German physicist, philosopher and experimental psychologist (1801–1887), professor at Leipzig and the founder of psychophysics, the discipline that first proposed measuring the relation between the physical world and mental sensation. Trained in medicine and then in physics, he spent his early years on electrical measurement and on the mathematics of the galvanic battery, and published in parallel, under the pseudonym Dr. Mises, satirical pieces such as the Comparative Anatomy of Angels — a double life of exact measurement and speculative humour that runs through everything he wrote.

In 1839 he collapsed into a crisis that lasted three years: partly blind, unable to eat or to bear light, he abandoned his chair and withdrew into a darkened room, and the recovery he described afterwards as a second birth turned him from physics towards the question of whether mind and matter are two faces of one thing. From that conviction came Nanna, or the Soul-Life of Plants (1848) and Zend-Avesta (1851), where he defended a panpsychist view of nature and the 'daylight view' — that consciousness is spread through the universe rather than confined to human skulls — against the 'night view' of a mechanical, mindless cosmos.

His decisive work, Elemente der Psychophysik (1860), grew from the intuition that the relation between a stimulus and the sensation it produces can be written as an equation: the Weber–Fechner law, which holds that sensation grows as the logarithm of the stimulus, gave psychology its first quantitative statement and the methods — limits, constant stimuli, average error — that laboratories used for a century.

His last major book, Vorschule der Ästhetik (1876), applied the same experimental spirit to beauty, asking not what art ought to be but which proportions people actually prefer. Wilhelm Wundt built his laboratory on Fechner's methods, William James called him a man of genius, and Freud took from him both the notion of psychic energy and the principle of constancy.