HYPOTHESES. 7 



than that the one is motion continuing in the same 

 direction, the other is motion which at each instant 

 changes its direction. There is, therefore, according 

 to the views I have stated, no absurdity in supposing 

 that all motion may be produced in one and the same 

 way; by the same kind of cause. Accordingly, the 

 greatest achievements in physical science have con- 

 sisted in resolving one observed law of the production 

 of motion into the laws of other known modes of pro- 

 duction, or the laws of several such modes into one 

 more general mode ; as when the fall of bodies to the 

 earth, and the motions of the planets, were brought 

 under the one law of the mutual attraction of all 

 particles of matter ; when the motions said to be pro- 

 duced by magnetism were shown to be produced by 

 electricity ; when the motions of fluids in a lateral 

 direction, or even contrary to the direction of gravity, 

 were shown to be produced by gravity; and the like. 

 There is an abundance of distinct causes of motion 

 still unresolved into one another ; gravitation, heat, 

 electricity, chemical action, nervous action, and so 

 forth ; but however improbable it may be that these 

 different modes of production of motion should ever 

 actually be resolved into one, the attempt so to resolve 

 them is perfectly legitimate. For though these various 

 causes produce, in other respects, sensations intrinsi- 

 cally different, and are not, therefore, capable of being 

 resolved into one another, yet in so far as they all 

 produce motion, it is quite possible that the immediate 

 antecedent of the motion may in all these different 

 cases be the same ; that the other causes may pro- 

 duce motion through the intermediate agency of 

 heat, for instance, or of electricity, or of some com- 

 mon medium yet to be discovered. 



We need not extend our illustration to other cases, 



