Gustav Theodor Fechner

Fechner’s early life
Gustav Theodor Fechner (1801-1887) was born in Germany. His father and grandfather were both religious pastors and when his father died, he went to live with an uncle. His father appreciated both science and religion. Fechner grew up with strong philosophical and religious interests but did not want to follow the family tradition. He started his medical training in Leipzig when he was sixteen, but never started a practice when he was finished. He started to translate French textbooks on physics and chemistry. He learned a lot, and went to conduct his own research in electricity, making him a teacher of physics at the University of Leipzig in 1824, he was made a full professor of physics in 1833. Fechner indulged his speculative side by studying nature-philosophy, a semi-scientific movement then popular in Germany. This movement regarded the entire universe as an organic entity complete with consciousness and other animate functions; at death, one’s individual consciousness presumably merges with the “over-consciousness” of the whole universe. Even as he recognized certain excesses in nature-philosophy (angels), Fechner also believed it offered an antidote to the rising tide of materialism that accompanied the increasing acceptance of physiological mechanism and its associated mechanistic worldview. While appreciating the potential scientific power of mechanistic analysis, Fechner also felt oppressed by its implications.

Fechner was upset by the apparent “two-facedness” of nature: the fact that the immutable materialistic laws governing the physical, external side of the world seemed to contradict, or be irrelevant to, the impression of free will that one actually experiences in consciousness. He became obsessed with the question “Does Nature or the world have a soul?” He had two anders to this question:
– Nachtansicht (“night view”) the universe as essentially a dead entity, with life and consciousness occurring only as incidental and fully predetermined by-products of mechanistic laws.
– Tagesansicht (“day view”) had roots in Leibniz’s monadology and took consciousness itself as the fundamental characteristic of a “besouled” universe, of which mechanistic laws offered only a partial, “external” view of reality.
For several years, Fechner fought a mental war between his night and day views, even as he successfully pursued his career in physics. In 1839, near the peak of his powers, he suffered a major breakdown. One day in October 1850, while lying in bed meditating, Fechner had an idea that eventually brought him back into the scientific mainstream—and to a position as one of the fathers of modern experimental psychology. His inspiration involved the relationships between the subjectively experienced intensities of various kinds of stimuli, and their actual objective strengths as measured physically.

The invention of psychophysics
Some simple, everyday observations about hearing and seeing can illustrate Fechner’s idea. If we hear, we take it for granted that little noise is filtered from a lot of background noise. As with vision, when a small light can be clearly visible in a dark room, but not in a brightly lit room. These facts indicate that conscious sensations of stimulus intensity are not perfectly reflect the physical reality, because the same stimuli create different impressions of their strength under different circumstances. These are examples of Kant’s point, namely, that the sensory system processes and transforms impressions from physical stimuli before bringing them to consciousness. Fechner thought it might be possible to measure the perceived as well as the physical intensities of sensory stimuli and then determine the mathematical relationships between the two measures. His intuition told him the relationships would turn out to be harmonious, and illustrative of the basic underlying unity of the psychological and physical worlds. Fechner called it psychophysics, the study of relationships between the objectively measured intensities of various stimuli and the subjective impressions of those intensities.

Fechner’s immediate practical problem was exactly how to measure the subjective intensities of stimulation. Fechner saw a solution to this problem, in his Leipzig friend and colleague, the physiologist Ernst Heinrich Weber. Weber had investigated people’s ability to distinguish between objects that looked the same but had different weights. He found that accurate discrimination depended on the relative rather than the absolute weight difference. The finest distinctions always involved weights that differed by at least 3 percent. Weber concluded that the just noticeable difference (jnd) for this particular discrimination task— the minimum amount of difference between two weights necessary to tell them apart—was always an amount approximately equal to 0.03 of the first of the two weights being compared. This work gave Fechner a crucial clue as to how he might empirically demonstrate an intrinsic harmony between the physical and the psychological. If one accepted that the jnd was in fact a constant fraction within each of the senses, then the jnd itself could be taken as the unit of measurement for subjective, psychological intensities of stimulation. One could then take the smallest intensity of a stimulus that can be perceived at all, a value Fechner called the absolute threshold, as the zero point on a scale of psychological intensities. Fechner recognized that these observed relationships between physical and subjective stimulus intensities for many different senses could be expressed by a formula: S=k log P (S: subjective intensity of a stimulus (jnd), P: physical intensity, k: constant). Although technically Weber’s law its mostly called the Fechner’s law. 

Criticism of Elements of Psychophysics
Critics of Fechner’s book “Elements of psychophysics” pointed to studies that showed that the equation was accurate only approximately and did not work for the extremes of high and low sensory intensities. They found that absolute limits differed from person to person, or even within the same person from time to time. Another criticism came from S. Smith Stevens (1906- 1973), who found that for some types of stimulation, subjective intensities increased more rapidly than the physical intensities – the opposite of what Weber and Fechner emphasized. He introduced  the power law (Stevens’ law). Like Fechner’s law, the power law provides only a rough approximation, holding most accurately for the middle ranges of physical stimulation and subject to certain fluctuations across individuals and situations.

Source: Fancher, R. E., & Rutherford, A. (2017). Pioneers of Psychology: A History (5th ed.). W.W. Norton And Company