308 ON SENSATION AND THE UNITY OF 



to conceive a material substance. We might have 

 a conception of time, but could have none of 

 extension, or of resistance, or of motion. And 

 without the three latter conceptions no idea of 

 matter could be formed. Our whole knowledge 

 would be limited to that of a shifting succession 

 of immaterial phenomena. But if an immaterial 

 substance may exist, it may have any conceivable 

 properties; and sensation may be one of them. 

 All these propositions may be affirmed with 

 complete dialectic safety, inasmuch as they cannot 

 possibly be disproved; but neither can a particle 

 of demonstrative evidence be offered in favour 

 of the existence of an immaterial substance. 



As regards the second hypothesis, it certainly 

 is not inconceivable, and therefore it may be true 

 that sensation is the direct effect of certain kinds 

 of bodily motion. It is just as easy to suppose 

 this as to suppose, on the former hypothesis, that 

 bodily motion affects an immaterial substance. 

 But neither is it susceptible of proof. 



And, as to the third hypothesis, since the logic 

 of induction is in no case competent to prove that 

 events apparently standing in the relation of 

 cause and effect may not both be effects of a 

 common cause that also is as safe from refuta- 

 tion, if as incapable of demonstration, as the other 

 two. 



In my own opinion, neither of these specula- 

 tions can be regarded seriously as anything but a 



