RELATION OF SENSATION TO STIMULUS 537 



satisfy its needs. It will have had to try many gustatory experiments 

 before, out of the sum of its material experiences, it will be able to 

 choose a number of like factors which can be grouped together as 

 ' sweet.' Judgment of quality of sensation involves a power of 

 abstraction and of classifying similar elements in different neural events 

 or reactions and the referring of these elements to the external world. 

 It is very difficult, however, to divest ourselves of the mental stand- 

 point reached as the result of many years' continual trials, successes 

 and failures, and constant care has to be exercised if we are not to 

 fall into the common conception of the ego, the personality, or soul, 

 as a sort of sentient god sitting somewhere in the brain, or, as Descartes 

 suggested, in the pineal gland, and receiving by means of one part or 

 other of his servile material brain a blue sensation from the eyes, 

 or an auditory impression, or a tactile impression, and then, if he feels 

 so inclined, pressing the stop in a pyramidal cell to let out a voluntary 

 motor response. An elementary unit in psychical life, as in neural 

 life, must be a complete reaction. It is from the reaction and not from 

 the sensation that a constructive psychology will have to be built up. 



Although any given sensation may be produced by many forms of 

 stimulation of the sense-organ, under normal circumstances each 

 sense-organ is so arranged and protected that it is only stimulated 

 by one kind of physical process, i.e. by the one for which its liminal 

 or threshold excitability is at a minimum. Thus the retina, though 

 it may be stimulated mechanically or electrically, is in the normal 

 individual very thoroughly protected from the possibility of such 

 excitation, so that all impulses arising in the retina may be almost 

 certainly referred to changes in the light- waves which fall on the 

 eye, and by means of the dioptric mechanism of this organ are thrown 

 in a distinct pattern upon the retina. Although the sensation is not 

 a reproduction of the stimulus, it is a symbol of the stimulus, and 

 can be used to inform us of events occurring in the world around. 

 Like stimuli, falling on the same end-organ, always evoke like sensa- 

 tions, other conditions being equal. An orderly sequence of sensa- 

 tions may therefore be interpreted as indicating a corresponding 

 orderly sequence of physical occurrence in the world around us. 



THE QUANTITATIVE RELATIONSHIPS BETWEEN STIMULUS 



AND SENSATION 



Since our sensations are merely symbols of the physical conditions 

 which give rise to them, it is important to inquire how far they corre- 

 spond quantitatively to differences in the energy of the afferent stimuli, 

 i.e. how alterations in the strength of stimulus will affect the intensity 

 of the resulting sensation. Whatever form of stimulus be applied and 

 whatever sense-organs be affected, a certain minimum intensity of 



