PSY'CHOPHYS'ICS (from Gk. 41,70), psyche, breath, life, soul c5vaiK6s, physikos, physical, from Oticis, physis, nature, from ch6eiv, phyein, to produce; connected with Lat. fui. I was, Skt. bluff, to become, and ultimately with Eng. be). The science of the inter-relations of mind and body. The term was coined by Fechner to desig nate an exact science of the relations of depend ence between the physical and psychical worlds. He discriminated an internal and an external psychophysics: in the former, sensation is con sidered in its direct relation to the brain and nervous system; in the latter, sensation is in directly studied in its dependence upon external physical stimuli. No definition could be more general. We have to ascertain the facts and laws of mind, and the physio logical facts and laws (of brain or of sense-organs) with which they are connected; we have to parallel the two series of events, noting how variation in the one is related to variation in the other; we have to express the functional interdependences exactly, i.e. in terms of measure ment, of mathematical formula.; and from our whole inquiry we shall attain a 'philosophical' standpoint, a theory of the general relation be tween the physical and psychical worlds.
Fechner is concerned, primarily, with the prob lem of mental measurement. And when we turn to the special portion of his Eleinente der Psycho physik, we find that he is occupied, for the most part, with the relation between stimulus and sensation that is formulated in Weber's law (q.v.). In the quantitative expression of this relation he sees the one universal psychophysical uniformity. A regrettable result of this general ization is that the term psychophysics has been narrowed in meaning. Thus Miinsterberg, finding his justification in historical usage, identifies it with the "question of the relation between stimu lus and sensation:" and the Century Dictionary defines it as "the science of the relations between stimuli and the sensations they evoke." Some writers go further still, and—in view of the fact that Weber's law has proved to be a law of sensation intensities—restrict. psychophysics to the intensive relations between stimulus and sensation. Thus Baldwin states that "investiga tions in intensity constitute psychophysics." In the light of modern developments, however, we have a full right to hold to the genera] definition laid down by Fechner.
A word must be said in detail of the psycho physical measurement methods, the elaboration of which owes much to Fechner, and the under standing of which is an integral part of in psychophysics. We cannot measure a mental process, sensation or other, as we can measure physical magnitudes; but we can measure func tion, sensitivity, and sensible discrimination: and we can measure the differences or distances that separate term from term within a sensation scale.
There are two groups of methods by aid of which such measurements can be made. The first in cludes the gradation methods, the second the error methods. In the former, we approach the object of measurement by slow degrees, in order to arrive at a single accurate result; in the latter, we approach the object to a certain distance over and over again, and submit the whole number of results to mathematical treatment, in order to arrive at the required determination. The grada tion methods imply, in general, a 'procedure with knowledge;' the observer knows the nature and the direction (increasing or decreasing) of the differences submitted to him for estimation; the error methods imply the 'procedure without knowledge'—the observer is left in ignorance of the conditions of the experiment. Typical of the latter class of methods is Vierordt's method of right and wrong cases. Two slightly different are laid before the observer. in a lung series of experiments; and he must say. in each case, wbether the sensation aroused by the sec ond stimulus is greater (louder, brighter, longer) than, equal to, or Tess than the sensation aroused by the first. He does not know either the amount of difference or the order of presentation of the two impressions in the separate tests. From the distribution of the `right,' wrong,' and 'equal' cases (right and wrong being determined by the objective relations of the stimuli), that stimulus difference is calculated which is able to evoke a just noticeable difference of sensation. Typical of the former class is Wundt's method of minimal changes. Two subjectively equal stimuli are taken, and the one gradually increased until it is just noticeably different from the standard. This point determined, the stronger stimulus is gradually decreased, until the point of subjective equality is reached. The differences from the standard found in these two determinations are averaged. The procedure is then repeated in the reverse direction; the one stimulus is decreased, to the point at which it is just noticeably less than the standard, and then increased again to subjective equality. . The two differences are averaged as Gefore. From the average of the two averages (the upper and lower difference limens), we obtain the value of the difference BIBLIOGRAPHY. Fechner, E/emente der PsychoBibliography. Fechner, E/emente der Psycho- physik (Leipzig, 1S60) ; G. E. :Midler, Zur Grund legung der Psychophysik (Berlin, IS7S); Mffinsterberg, Britrage cur ezycrimcntvllen Psychol ogie (Freiburg, I890) ;Ipe, Outlines of Psy chology (trans., London, 1S9.1) ; .1rehir schichte der Philosophic, vol. vi. (Berlin, 1893) Physiologische Psychologic (Leipzig, .1893) ; Sanford, Course in Exporimental Psychol ogy (Boston, 1898). See DISCRIMINATION, SEN SIBLE; LIMEN.