n. The science of the relations between stimuli and the sensations which they evoke.n. Psychophysics was defined by Fechner in 1860 as “an exact science of the functional relations or relations of dependency between body and mind, or more generally between the bodily and mental, the physical and psychical world”; and Wundt, in 1902, declares in the same spirit that psychophysics is to be understood as “an investigation of the relations that may be shown empirically to obtain between the psychical and the physical aspects of vital processes.” It may, however, be questioned whether a definition of this generality can be made useful for scientific purposes. It is, no doubt, implied in such phrases as ‘the psychophysical organism,’ by which we mean the correlated body-mind of actual experience, the ensouled body or the embodied mind, and ‘psychophysical evolution,’ by which we mean the evolution of this correlated body and mind. Nevertheless, a science cannot remain poised between two existing sciences; it must have a positive content of its own. In other words, psychophysics, viewed in this very general way, must soon show a tendency to fall either toward the side of psychology or toward that of biology, and to be subsumed under the one or the other of the sciences whose methods and results it is supposed to relate and combine. It was, perhaps, by an implicit recognition of this danger, reinforced by the desire to hold fast to all that might be tenable in the Fechnerian definition, that the meaning of psychophysics was, until quite recently, narrowed down to that special field of research in which Fechner had shown himself especially active—to the correlation of intensity of external stimulus with intensity of sensation. In this sense we speak of the four classical methods (least differences, right and wrong cases, average error, mean gradations) as ‘the psychophysical methods,’ and of Weber's Law as ‘the psychophysical law.’ It is needless to say that Fechner would never have assented to this restriction; that apart, however, the new definition is so obviously artificial, and the delimitation of subject-matter which it suggests is so obviously accidental, that it has neither logical standing nor prospect of survival. Indeed, it has at no time found acceptance among psychophysical workers: at the very least, the experiments made upon simple and compound reactions, and upon what is still known as the ‘time sense,’ have been also included under psychophysics. Of late years, the term has taken on a better and a broader meaning, a meaning which preserves the spirit if not the letter of Fechner's definition, and which promises to settle down into something like finality. Psychophysics may now be defined as that department of experimental psychology which aims, not at introspective analysis, but rather at the determination of the quantitative norms of the mental life. Thus, the reaction experiment is a psychological experiment if it is made with a view to the introspective analysis of the action-consciousness; it is a psychophysical experiment if its object is the determination of the time-values of certain typical organic reactions. In the former case, it can be performed only by trained students of psychology; in the latter, it may be made a means to the comparison of the capacities of children, the lower races of man, and even the higher animals. So the time-sense experiment is a psychological experiment, if our aim is the discovery of the conscious basis or vehicle of the time-consciousness; a psychophysical experiment, if we wish to establish the norms of temporal discrimination, or to institute comparative studies of the time-discrimination of different ages and races. Psychophysics has fallen to the one side—to the side of psychology: for the establishment of quantitative norms of the mental life must be intrusted to those who have made mind their special study. At the same time, when the norms have once been established, there is no further need of introspection; psychophysics becomes a matter of technique and of external observation; and, in so far, the Fechnerian view is retained. It may be added that the results of psychophysics, while they must be obtained by the experimental psychologist working, at any rate in the first instance, within the psychological laboratory, are of great importance both for anthropology and for medicine; and that the methods, onoe worked out, may be carried a field and thus applied under conditions widely remote from those of their first elaboration. See experimental psychology.
