RELATION OF SENSATION TO STIMULUS 4$) 



ing from some part of the body or from some region outside of the body. 

 Thus the sensation of taste is always localised in the mouth ; sensation of 

 touch at the skin or surface of the body ; while the sensations of hearing 

 and of sight are ' projected,' i.e. are interpreted as coming from the en- 

 vironment outside ourselves. Even the organic sensations of posture or 

 fatigue are referred to the peripheral reacting parts of the body and not to 

 the central nervous system. A sensation therefore cannot be interpreted as 

 a reproduction of external events, but as a symbol of these events evoked by 

 stimulation of the sense-organs of the body. 



It is indeed impossible by a purely intuitive study of sensations to arrive 

 at any correct idea of their origin or of the factors concerned in their produc- 

 tion. No sensation is the immediate and sole product of a stimulus applied 

 to the peripheral end of a nerve fibre, but the simplest sensation involves I 

 a judgment, i.e. complex neural activities which are the resultant of innumer- 1 

 able past and present streams of nervous impulses aroused by peripheral 

 events and poured into the central nervous system. It is important therefore 

 not to regard a sensation as in any way constituting an elementary unit, by 

 the aggregation of a number of which a conscious state is produced. As we 

 have seen, the primitive function of the whole nervous system is reaction. 

 The neural life of an animal is composed of a series of reactions, some simple, 

 some complex, and becoming ever more complicated as we ascend the animal 

 scale. The first reactions of a baby, for instance, will be those by which it 

 procures nourishment and satisfies a need. The earliest event in its dawning 

 consciousness will be, not a sensation of sweetness or of colour, but that of a 

 thing which can 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, how- 

 ever, to divest ourselves of the mental standpoint 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 pyramida cell to let out a volun- 

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

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

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

 Although any given sensation may be produced by many : 

 stimulation of the sense-organ, under normal circumstances each sense 

 is so arranged and protected that it is only stimulated by one kind of ph; 

 process, i.e. by the one for which its liminal or threshold excitability 11 

 nmrimum. Thus the retina, though it may be stimulated mechanic 



