452 ON SENSATIONS IN GENERAL 



in sensations which are more or less present to the mind. Sensations by which 

 we have knowledge of the position in space of our bodies and their members, 

 also of the extent of their movements and the intensity of muscular contrac- 

 tions in short, all those sensations which are comprehended as belonging to 

 the sense of motion come under this class. The sensations from other internal 

 organs like the heart, stomach, intestines, bladder, etc., also belong here. 

 The latter are not sharply denned unless intensified by some special cause; 

 then they sometimes become very painfully conspicuous. As a rule, however, 

 they are wholly indefinite and contribute in consciousness only toward the 

 general state of feeling, which not only varies greatly according to the nature 

 of these vague sensations, but may very profoundly influence our whole being. 



One of the most significant facts in connection with the physiology of 

 sensation is that our conscious sensations do not arise in the organs to which 

 the afferent nerves are distributed and to which the stimuli are applied. The 

 sensation of sight, for example, is not in the eye, the sensation of sound is 

 not in the ear, etc. The peripheral sense organs and the peripheral endings 

 of afferent nerves in general are for the sole purpose of transferring the stimuli 

 which strike them to the appropriate nerves. The nerves transmit the excita- 

 tions thus aroused to the central organs of the nervous system and the con- 

 scious sensation arises only by the activity occasioned in the brain. Different 

 parts of the brain are set in action directly, according as one afferent nerve or 

 another is excited (cf. Chapter XXIII). 



How can a material change in the brain give rise to a conscious sensation ? 

 Philosophers of all times have tried to answer this question. Since we are 

 discussing it here only from the standpoint of natural science, we cannot 

 enter into the philosophical considerations. It is likely, indeed, that the 

 question can never be answered from the standpoint of natural science alone, 

 for, as Du Bois-Reymond especially has pointed out, the question is at bottom 

 a metaphysical one. 



If our knowledge of nature were so far advanced that all the movements in 

 the world could be resolved into the movements of atoms, and our explanation 

 of nature could thus be reduced to the mechanics of atoms, we would of course 

 be in a position to describe the material changes taking' place in the brain in 

 definite psychical processes very exactly. Satisfactory as this knowledge would 

 be, it would nevertheless be unable to give us any conclusive information con- 

 cerning the relation of such movements to such ultimate facts as : " I feel com- 

 fortable," " I feel pain," and the proposition immediately deducible therefrom : 

 " I think, therefore I am." That is to say, it is impossible to conceive scien- 

 tifically how consciousness and thought can arise out of the interplay of atoms. 

 Indeed we could imagine a world similar to our own, in which everything would 

 take place exactly as in our world, but where there were no consciousness and 

 no thought; and yet the mechanics of atoms would be just as valid for such a 

 world as for our own. 1 



In what sense do our sensations produced by external stimuli correspond 

 to reality? Philosophy and natural science attack this problem from opposite 



1 Cf. Du Bois-Reymond, " Limits of Our Knowledge of Nature," translated by J. Fitz- 

 gerald in Popular Science Monthly, May, 1874. 



