PSYCHOPHYSICS, the science that deals with the deter mination of the relation between the mental and physical worlds, or, more specifically, of the relation between sensation and its stimulus. The word is generally used with reference to the quan titative determination of the latter relationship.
History.—Psychophysics was established by Gustav Theodor Fechner (iSoi–I887), who coined the word, invented the three fundamental methods, conducted elaborate psychophysical experi ments, and began a line of investigation that still persists in experi mental psychology to-day. Fechner's classical book, Die Elemente der Psychophysik (186o), may be looked upon as the beginning, not only of psychophysics, but also of experimental psychology (q.v.) itself. Fechner, trained in physics, became interested in his later life in metaphysics, and cast about for a way of relating the spiritual to the material world. He hit upon the notion of measur ing sensation in relation to its stimulus. Ernst Heinrich Weber (1 ) , the physiologist, had discovered that the just notice able differences between the intensities of sensation always bore, when stated in the increment of stimulus necessary to produce such differences, an approximately constant ratio to the total magnitude of the stimulus. This fact, properly speaking, is Weber's Law : if two weights differ by a just noticeable amount when separated by a given increment, then, when the weights are increased, the increment must be proportionally increased for the difference to remain just noticeable. Fechner chanced upon Weber's Law and undertook to use it for the measurement of sensation. If R be the stimulus (Rein), and S be the resultant sensation, and A signify an increment of either, then Weber's Law becomes AR/R =a constant, for the just noticeable difference. Fechner went further and assumed that all equal increments of sensation must be pro portional to the same ratio, AR/R, that is to say AS = cAR/R, where c is a constant of proportionality. If this equation is inte grated, if R be measured in terms of the threshold stimulus (the value of stimulus at which S is zero or just ready to appear), and if the constant be changed to k for common logarithms, we have S= k logR.
This particular formula Fechner named Weber's Law, although it is really Fechner's Law and is thus sometimes called the Fech ner-Weber law. It expresses the simple relation that the magnitude of a stimulus must be increased geometrically if the magnitude of sensation is to increase arithmetically. For Fechner it meant that the relation between the spiritual and material worlds is statable, and that there is therefore only one world, the spiritual; but for physiologists and for many philosophers it meant the measuring of sensation in relation to a measured stimulus, and thus the possibility of a scientific quantitative psychology. Fech ner got his conception of psychological measurement from Herbart, but he was really refuting Herbart in demonstrating that psychology can be experimental.
Fechner's original work stimulated much research and much controversy. It was argued against him that it is introspectively obvious that sensations do not have magnitude (the quantity objection), that "a scarlet is not just so many pinks." This diffi culty was met by the Belgian, J. L. R. Delboeuf (1831-1896), who developed the concept of the sense-distance, holding that sensations, although not complex magnitudes, are separated by variable distances which can be compared as "greater," "equal," or "less," the three categories of judgment which the psycho physical methods require. In Germany, Georg Elias Muller (1850– ) undertook an elaborate criticism of Fechner and an extension of his work. In America, E. B. Titchener (1867-1927) made a historical and practical exposition of psychophysics in the second volume of his Experimental Psychology (1905). More recently F. M. Urban, then also in America, improved one of the methods and developed the concept of the psychometric function. Other well known psychophysicists are W. Wirth in Leipzig, and William Brown and G. H. Thomson in England.