STRUCTURE OF SENSIFEROUS ORGANS 307 



the metaphysical problem of problems, and the 

 solutions which have been suggested have been 

 made the corner-stones of systems of philosophy. 

 Three mutually irreconcilable readings of the 

 riddle have been offered. 



The first is, that an immaterial substance of 

 mind exists; and that it is affected by the mode 

 of motion of the sensorium, in such a way as to 

 give rise to the sensation. 



The second is, that the sensation is a direct 

 effect of the mode of motion of the sensorium, 

 brought about without the intervention of any 

 substance of mind. 



The third is, that the sensation is, neither 

 directly nor indirectly, an effect of the mode of 

 motion of the sensorium, but that it has an 

 independent cause. Properly speaking, therefore, 

 it is not an effect of the motion of the sensorium, 

 but a concomitant of it. 



As none of these hypotheses is capable of even 

 an approximation to demonstration, it is almost 

 needless to remark that they have been severally 

 held with tenacity and advocated with passion. I 

 do not think it can be said of any of the three 

 that it is inconceivable, or that it can be assumed 

 on a priori grounds to be impossible. 



Consider the first, for example; an immaterial 



substance is perfectly conceivable. In fact, it is 



obvious that, if we possessed no sensations but 



those of smell and hearing, we should be unable 



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