x 4 o SCIENTIFIC METHOD IN PHILOSOPHY 



the earlier and later parts of one perceived motion are 

 distinguished by the less and greater vividness of the 

 sensations. 



This answer shows that physiology can account for our 

 perception of motion. But physiology, in speaking of 

 stimulus and sense-organs and a physical motion distinct 

 from the immediate object of sense, is assuming the truth 

 of physics, and is thus only capable of showing the 

 physical account to be possible, not of showing it to be 

 necessary. This consideration brings us to the psycho- 

 logical answer. 



(2) The psychological answer to our difficulty about 

 motion is part of a vast theory, not yet worked out, and 

 only capable, at present, of being vaguely outlined. We 

 considered this theory in the third and fourth lectures ; 

 for the present, a mere sketch of its application to our 

 present problem must suffice. The world of physics, 

 which was assumed in the physiological answer, is ob- 

 viously inferred from what is given in sensation ; yet as 

 soon as we seriously consider what is actually given in 

 sensation, we find it apparently very different from the 

 world of physics. The question is thus forced upon us : 

 Is the inference from sense to physics a valid one ? 

 1 believe the answer to be affirmative, for reasons which I 

 suggested in the third and fourth lectures ; but the answer 

 cannot be either short or easy. It consists, broadly speak- 

 ing, in showing that, although the particles, points, and 

 instants with which physics operates are not themselves 

 given in experience, and are very likely not actually exist- 

 ing things, yet, out of the materials provided in sensation, 

 it is possible to make logical constructions having the 

 mathematical properties which physics assigns to particles, 

 points, and instants. If this can be done, then all the 

 propositions of physics can be translated, by a sort of 



