36 GENERAL PHYSIOLOGY 



our own minds. Any doubt as to the existence of a physical 

 world outside of mind, will appear absurd to one who has not 

 reflected upon it : a body, e.g., a stone, a tree, a man, which we 

 look upon, really exists, no one will deny this ; we actually see the 

 body, others see it ; and we say it exists. We are right ; without 

 a doubt it exists, but it does not exist outside our mind; for, 

 when we examine carefully the grounds for speaking as we do, 

 we find that what we believe we see or feel as a body outside 

 our mind is actually something quite different. 



Let us prove this. We have created our knowledge of a 

 physical world by means of sense-perception. The question as to 

 what can and does give us this knowledge is, therefore, one belong- 

 ing to the physiology of the senses. Now the physiology of the 

 senses shows that all that comes in through the door of our senses 

 affords us, simply and solely, sensations. The many features that 

 constitute the image of a body, e.g., a piece of gold, are so many 

 different sensations, e.g., a yellow colour, hardness, weight and cold- 

 ness. Persons with an innate defect in a sense, in whom a certain 

 group of sensations is not mediated, e.g., persons born blind, have, 

 therefore, an idea of the physical world that is wholly different 

 from that of normal persons. This is clearest in those interesting 

 cases in which persons who are born blind and have constructed 

 their physical world solely by means of the senses of touch, 

 hearing, smell, taste, etc., have been made to see by surgical 

 operations. If objects that such persons have often had in their 

 hands be brought for the first time before their eyes without 

 their examining them by the other senses, e.g., by touching, they 

 do not recognise them: a ball appears to them as something 

 wholly new, and only when they touch it do they realise to their 

 surprise its identity. At that moment a new world begins to 

 arise in them. The physical world depends, therefore, wholly 

 upon the development of our sense-organs ; to animals with sense- 

 organs developed differently from ours it must appear very different, 

 in proportion as they receive other sensations. With our death, 

 with the destruction of the senses and the nervous system, the 

 physical world in its previous form disappears. 



These facts are of far-reaching significance. They show that what 

 appears to us as matter is in reality our own sensations, or 

 ideas, our own mind. When I see a body or perceive it by means 

 of my other senses, in reality I have not a body outside of myself 

 but only a number of sensations in my mind. Beyond these I 

 know nothing concerning it and can only form hypotheses. 



It is necessary that we accustom ourselves to this fundamental 

 truth, and that we get rid of the error of the existence of a physical 

 world outside of mind. In order to facilitate this let us consider 

 the consequences of this truth. 



If the physical world is only my own sensation, or, better, since 



