PHYSIOLOGICAL PSYCHOLOGY 141 



existence. Of course, a developed psychology would endeavor to extend 

 this plan to the entire range of consciousness. Fechner, however, con- 

 fines himself to a single fundamental point — the relation between 

 stimulus and sensation as generalized in Weber's law ; although, just as 

 Lotze before him, he considers other questions in a most suggestive 

 manner, such, notably, as the seat of the soul, sleep and reminiscence. 

 In pursuance of his early conviction,' that soul and body are but opposite 

 sides of an identical existence (conscious), he took it for granted that 

 their reciprocal action would be proportional. But this was belief, not 

 science. Weber's work led Fechner to test the hypothesis, that the in- 

 crease of physiological excitation holds the key to psychological changes. 

 And his interest was stimulated by the fact that, if this could be proved 

 accurately, his philosophy would benefit by so much indubitable evi- 

 dence. Consequently, he was moved to verify Weber's law by numerous 

 experiments, chiefly of a physical sort. Sensations of pressure and 

 muscular effort, detected by the use of weights ; sensations of tempera- 

 ture, determined by cold and hot water; sensations of light, handled 

 by the photometer; and sensations of sound, observed by reference to 

 falling bodies, all tended to confirm the same general relation between 

 stimulus and the psychological event. Given what Herbart called a 

 " threshold of sensation," and having fixed this as a constant for each 

 class of sensation, Fechner found it possible to infer, by strict induc- 

 tion, that the intensity of the sensation is equal to the proportion of the 

 stimulus, multiplied by the logarithm of the excitation, divided by the 

 threshold of stimulus. In other words, we can obtain a formula for 

 the quantitative relation of physical and psychological events consid- 

 ered as magnitudes. This formula, which provides a means of meas- 

 urement, declares that the sensations increase proportionally to the 

 logarithm of the stimulus. 15 As a law, Fechner affirms dogmatically 

 that it applies for internal (psychological) states and, within limits, 

 reasons for which can be given, for external (physiological or physical) 

 conditions. The result was obtained by three methods. (1) The 

 Method of Differences which are Just Observable. This means that 

 the operator finds, first, the least greater or the least smaller stimulus 

 which can just be sensed as different by the subject; and then proceeds 

 to add increments to this, or, inversely to subtract increments from it, 

 till the intensity or diminution come into clear recognition. Divide 

 the sum of the initial and the altered stimulus and you arrive at the 

 differential of sensibility. (2) The Method of True (Eight) and 

 False (Wrong) Cases. Here the operator applies two stimuli, which 

 differ slightly, to the subject, and inquires whether the first is greater 



15 Cf. "German Psychology of To-day," Ribot, pp. 138 f. (Eng. trans.); 

 " Outlines of Psychology," Kiilpe, pp. 164 f. ; Ward in Mind, Vol. I. (old series), 

 pp. 452 ff. ; ©r in any standard psychology, e. g., James or Wundt or Ladd. 



