Editing Gustav Fechner (section)

=== Elemente der Psychophysik ===

Fechner's epoch-making work was his ''Elemente der Psychophysik'' (1860). He started from the [[Monism|monistic]] thought that bodily facts and conscious facts, though not reducible one to the other, are different sides of one reality. His originality lies in trying to discover an exact mathematical relation between them. The most famous outcome of his inquiries is the law known as ''Fechner's Law'', which may be expressed as follows:<ref name="EB1911"/>

:"In order that the intensity of a sensation may increase in arithmetical progression, the stimulus must increase in geometrical progression."

The law has been found to be immensely useful, but to fail for very faint and for very strong sensations. Within its useful range, Fechner's law is that sensation is a logarithmic function of physical intensity. [[Stanley Smith Stevens|S. S. Stevens]] pointed out that such a law does not account for the fact that perceived relationships among stimuli (e.g., papers coloured black, dark grey, grey, light grey, and white) are unchanged with changes in overall intensity (i.e., in the level of illumination of the papers). He proposed, in his famous [[Stevens' power law|1961 paper]] entitled "To Honor Fechner and Repeal His Law", that intensity of stimulation is related to perception via a power-law.

Fechner's general formula for getting at the number of units in any sensation is ''S'' =&nbsp;''c''&nbsp;log&nbsp;''R'', where ''S'' stands for the sensation, ''R'' for the stimulus numerically estimated, and ''c'' for a constant that must be separately determined by experiment in each particular order of sensibility. Fechner's reasoning has been criticized on the grounds that although stimuli are composite, sensations are not. "Every sensation," says [[William James]], "presents itself as an indivisible unit; and it is quite impossible to read any clear meaning into the notion that they are masses of units combined."<ref name="EB1911"/>