SOURCES OF THE NEW PSYCHOLOGY. 101 



" feels " heat is, from a physiological point of view, an indirect 

 question. Physiologically, the nerves of the skin may respond to 

 heat by some chemical process ; that they do so respond may be 

 inferred on the hypothesis of a correspondence between the occur- 

 rence of a sensation of heat and the action of the nerve. The di- 

 rect question is one of psychology ; it is asked by physiology for 

 its own purposes, and the psychological data are collected as long 

 as they are of use in this way. Physiology, however, is " physics 

 and chemistry of the body," and as soon as psychological data 

 cease to afford physical deductions the interest of the physiologist 

 generally ceases. The study of the psychology of sensation and 

 action, however, has formed and still forms an important portion 

 of physiology. 



Historically considered, the study of the sensations of the skin 

 received its first great impulse from Ernst Heinrich Weber's 

 monograph, Tastsinn und Gemeingefiihle* This has been fol- 

 lowed by the work of a host of investigators from the labora- 

 tories of Ludwig, Du Bois-Reymond, and their pupils. f 



The physiology of the eye originated much of the psychology of 

 sight. Concerning the functions of the optical system, physiology 

 can scarcely be said to have gone beyond the dioptrics of the eye. 

 Nearly all further knowledge consists of deductions from the 

 mental experiences of the subject. For example, physiology 

 knows almost nothing concerning the functions of the retina. 

 Psychologically, however, the color sensations and their combi- 

 nations can be accurately measured. It is true that the investi- 

 gations of color vision have been and are mainly carried out by 

 physiologists and physicists ; but the point of view has become 

 primarily a purely psychological one. This is strikingly exempli- 

 fied in the researches of Konig, from which physiological deduc- 

 tions are practically excluded. For the various other phenomena, 

 such as those of the optical illusions, of monocular and of binocu- 

 lar space, we have at present no hope of anything beyond a 

 psychological knowledge, and the investigations of Hering, Helm- 

 holtz, and others can be regarded as direct contributions to psy- 

 chology. 



There is a third science whose influence is to-day the strongest 

 of all. Physics is theoretically the co-ordinate science to psychol- 

 ogy. Every direct experience has an objective, or physical, and 

 a subjective, or psychical, side. Again, the fundamental science 

 of Nature is physics, that of Mind is psychology. Practically, 

 however, psychology receives from the most powerful science 



* Wagner's Handworterbuch d. Physiologie, 1851, vol. iii (2), p. 561 ; also separate. 



\ For summaries and references, see Funke und Hering, Physiologie der Hautem- 

 pfindungen und der Gemeingefiihle, Herrman's Handbuch der Physiologie, 1880, vol. iii ('2), 

 p. 287; and Beaunis, Nouveaux elements de physiologic humaine, vol. ii, Paris, 1888. 



