THE LIMITATIONS OF THE SENSES 341 



of conceiving new kinds of properties as a congenitally deaf man is 

 of imagining sound or a congenitally blind man colour which a 

 blind man once imagined to be like the sound of a trumpet. Men 

 are sometimes paralysed as regards sensation and movement, that 

 is, they lose in some parts of their bodies the tactile and muscular 

 senses. Conceivably they might be born and might exist entirely 

 without them. Judging from the analogy of congenitally blind 

 and deaf men, if I had never possessed any senses but those of 

 smell, taste and hearing, I could have no idea of extension. So 

 slowly did I develop mentally after birth that apparently I grad- 

 ually built up my idea of it. It was, therefore, not an intuition, 

 an instinctive bit of knowledge, but an acquirement, an induction 

 from experience. And, if my notion of space was not an intui- 

 tion, it is hard to believe, so inconceivable does existence which is 

 not spatial now seem to me, that I have any other intuitive ideas. 

 Moreover, as we shall see later, all ideas are founded on memory, 

 and, therefore, cannot be intuitive or instinctive. An instinct is a 

 mere impulse to action. 1 



574. On the other hand, I have no doubt that, if I could see the 

 colours that lie beyond the red and violet ends of the spectrum, I 

 could think in terms of them. Probably, therefore, if a new power 

 of perception, analogous to sight or hearing for instance, were 

 suddenly conferred on me, my intellectual powers would suffice to 

 enable me to use the materials provided by it for purposes of 

 thought. 



575. It is certain that our senses, even when at their best, are 

 not sufficient to enable us to perceive completely all the properties 

 of objects ; thus we can see only within a certain range of the 

 spectrum and smell odours and taste flavours of a certain intensity. 

 It is practically certain that we cannot perceive at all some of the 

 properties of objects ; thus, apparently, a bat has some sense lack- 

 ing to us which enables it to perceive some property unknown to 

 us, for it is said to be able to avoid obstacles when flying in what 

 appears to us absolute darkness. Again, we are unable to perceive 

 that property in material bodies which acts across apparently empty 

 space, and the effects of which we term gravitation. We are able 

 to perceive only how it affects the relations of material bodies to 

 one another. It is possible that all the properties of some real 

 things are altogether outside the range of our perceptions, and, there- 

 fore, that some real things exist of whose existence we cannot be 

 aware (at least directly) whose properties are inconceivable to us, 



1 See ,618 etseq. 



