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Elemente der Psychophysik
Gustav Theodor Fechner (1860)
Robert H. Wozniak
© 1999 Robert H. Wozniak. All rights reserved. Previously published in Wozniak, R. H. (1999). Classics in Psychology, 1855-1914: Historical Essays. Bristol, UK: Thoemmes Press. Reprinted by permission of the author.
The emergence of laboratory psychology in the 19th century required two types of innovation. One involved the development of apparatus and methods for the control and systematic variation of stimuli and the precise registration of response; the other involved the creation of methods for the quantitative measurement of mental processes. While early apparatus was the work of many hands, the first methods, rationale, and systematic use of mental measurement was essentially the contribution of one person, Gustav Theodor Fechner.
Although Fechner was originally trained as a physicist, the basis of his interest in mental measurement was far more metaphysical than scientific. Committed to panpsychism, the notion that all nature is besouled ("beseelt"), and rejecting the Cartesian dualism of mind and body, Fechner adopted a dual-aspect monistic view of the relationship between the psychical and the physical.
Dual-aspect monism holds that mind and body are two aspects of one and the same existent. Just as a curved line can be characterized at every point by both concavity and convexity, all nature, Fechner argued, can be as readily viewed from the psychical as from the physical perspective. The psychical and the physical, in other words, are the dual aspects under which nature appears in experience.
Given nature's two-sidedness, the question then arises as to the functional relationship that exists between its psychical and physical aspects. In addressing this question, Fechner worked out the program of psychophysics. As he himself described it: "The task did not at all originally present itself as one of finding a unit of mental measurement; but rather as one of searching for a functional relationship between the physical and the psychical that would accurately express their general interdependence."
To achieve this goal, however, Fechner had to find a way to measure the intensity of mental process; and this presented a very significant problem. Unlike physical processes, which are external, public, objective, and open to direct measurement, mental processes are internal, private, subjective, and cannot be measured directly. Somehow, an indirect method had to be developed.
According to Fechner's own account, it was on the 22nd of October 1850 that he arrived at an insight that would provide the solution to this problem. Relative increase in mental intensity, he realized, might be measured in terms of the relative increase in physical energy required to bring it about. This insight, in effect, defined the psychophysical program; and for the next ten years Fechner devoted himself to developing measurement methods, gathering data on the psychophysics of lifted weights, visual brightnesses, and tactual and visual distances, and systematizing the mathematical principles underlying his work. In 1860 he published the results of his ten-year effort in one of experimental psychology's most original monographs, the Elemente der Psychophysik.
The Elemente consisted of two volumes. The first volume was devoted to what Fechner called "outer psychophysics," the study of the functional relationship between increase in physical stimulus magnitude and increase in sensation. Here he described three probabilistic methods for the collection of psychophysical data, marshaled a great deal of evidence in support of the existence of a logarithmic relationship between the intensity of sensation and the intensity of the stimulus, and spelled out the basic assumptions of psychophysics.
As with any form of measurement, psychophysical measurement requires the establishment of a zero point and a basic measurement unit. To define a zero point, Fechner borrowed the concept of "limen" from Herbart. Below a certain stimulus intensity, there is no sensation. This is the absolute limen or threshold of sensation. At the limen, sensation is assumed to be zero. In arriving at a unit of measurement, Fechner took the notion of a "just noticeable difference" from Weber's earlier experiments on lifted weights. A just noticeable difference is the minimum reportable difference in intensity of sensation brought about by a minimal change in physical stimulus intensity.
Fechner's own contribution was to recognize that the just noticeable difference could be made the basic unit of measurement of the intensity of sensation. Assuming that sensation is zero at the limen and that all just noticeable differences are equal regardless of where on the scale of physical intensity they fall, a given sensation can be said to be some number of just noticeable differences above zero or above or below another sensation. The magnitude of sensation, in other words, can be scaled in relation to the scale of physical intensity.
The goal of psychophysics was not, of course, the measurement of specific sensations but the development of a general law for the functional relationship between the psychical and the physical. Starting from the empirically derived observation that a just noticeable difference requires a relatively small change at low levels of stimulus intensity and relatively larger changes as intensity increases (or more specifically that the ratio of change in intensity to level of intensity is a constant for any given just noticeable difference) and with the help of a few additional assumptions including the assumption that the intensity of sensation is zero at the limen, Fechner derived the law that bears his name, viz., that the intensity of sensation equals a constant times the logarithm of the intensity of the stimulus. This was "outer psychophysics."
In the second volume of the Elemente, Fechner went on to address "inner psychophysics," the nature of the functional relationship between the intensity of sensation and the magnitude of nervous activity in the brain. In this analysis, of course, he was seriously restricted by the inaccessibility of nervous process and the relatively undeveloped state of brain science; but true to his metaphysic, he concluded that the relationship between physical brain process and psychical sensation must, like that between physical stimulus and psychical sensation, be logarithmic in form.
In developing psychophysics, Fechner had succeeded, at least to his own satisfaction, in specifying the functional relationship between the intensity of sensation, the psychophysical unity viewed from the mental side, and the intensity of the stimulus, the psychophysical unity viewed from the material side. In the course of pursuing this effort, however, he had incidentally introduced mental measurement into psychology, recognized the inherently probabilistic nature of that measurement in the development of his three measurement methods, and carried out psychology's first extensive, fully programmatic experimental research effort. It is perhaps little wonder, therefore, that historians have been known to date the birth of scientific psychology from the appearance of Fechner's Elemente.
 1801-87. For biographical information on Fechner, see Lasswitz, K. (1896). Gustav Theodor Fechner. Stuttgart: Frommann.
 The nature and development of Fechner's rather unusual mind/body position is nicely described in Woodward, W.R. (1972). Fechner's panpsychism: A scientific solution to the mind-body problem. Journal of the History of the Behavioral Sciences, 8, 367-86.
 Fechner, G.T. (1860). Elemente der Psychophysik. Leipzig: Breitkopf und Härtel, 2, p. 559 (Reprinted, Bristol: Thoemmes Press, 1999).
 Ibid. Only the first volume has been translated into English; it appeared as Fechner, G.T. (1966). Elements of Psychophysics. Volume I. New York: Holt, Rinehart and Winston.
 Herbart, J.F. (1824-5). Psychologie als Wissenschaft, neu gegründet auf Erfahrung, Metaphysik und Mathematik. Königsberg: Unzer.
 Weber, E.H. (1834). De pulsu, resorptione, auditu et tactu. Annotationes anatomicae et physiologicae. Lipsiae: Koehler.
 See, for example, Boring, E.G. (1950). A History of Experimental Psychology (2nd ed.). New York: Appleton-Century-Crofts, p. 275.