THE CONCEPTUAL WORLD 43 



having properties and being subject to transformations. 

 Thus we apply the category of substance. 



Has this universe that we construct from the data 

 of sensation objective reality ? We are led quite 

 naturally by our study of physiology to the notion of 

 idealism. We see that our perception of things, that 

 is, our knowledge of the universe, depends on the 

 integrity of functioning of certain bodily structures, 

 and upon the condition that in men in general this 

 integrity of functioning is normal, that is, common to 

 the great majority of mankind. 



To say that a thing exists is to say that it is perceived 

 in some \vay ; that immediately or remotely it affects 

 our state of consciousness. To say that the star 

 Sirius exists is to say that the stimulation of the retina 

 by a minute spot of light transmits certain molecular 

 disturbances along the optic nerve, and that other 

 molecular disturbances are set up in the tissues of the 

 central nervous system. Even if we do not see those 

 dark stars that we know to exist, there are still evidences 

 of their being that in some way affect the instruments 

 of the astronomer and lead to their being perceived. 

 Even if we do not actually see the emanations from a 

 radio-active substance, we can cause these emanations 

 to produce changes in something that we can see. We 

 speak of the star as a minute spot of coloured light. 

 But if we are short-sighted the spot becomes a little 

 flare, and if we are colour-blind the hue of the star is 

 different from what it is to normal persons. If we put a 

 drop of atropine into one eye and then close the other, 

 objects appear to lose their distinctness, but if we close 

 this eye and then open the other, the original sharpness 

 of vision returns. When we are bilious, wisps and spots 

 may appear on a sheet of white paper that at other 

 times was blank. If we take an overdose of quinine, 



