288 ixDUCTiOiV. 



The ideal limit, therefore, of the explanation of natural phenomena 

 (towards Avhich as towards other ideal limits we are constantly tendino-, 

 without the prospect of ever completely attaining it,) would be to show 

 that each distinguishable variety of our sensations, or other states of 

 conscioTisness, has only one sort of cause; that, for example, whenever 

 we perceive a white color, there is some one condition or set of con- 

 ditions which is always present, and the presence of which always 

 produces in us that sensation. As long as there are several kno%\Ti 

 modes of production of a phenomenon, (several different substances, 

 for instance, which have the property of whiteness, and between 

 which we cannot trace any other resemblance) so long it is not im- 

 possible that one of these modes of production may be resolved 

 into another, or that all of them may be resolved into some more 

 general mode of production not hitherto recognized. But wiien the 

 modes of production are reduced to one, we cannot, in point of sim- 

 plification, go any further. This one may not, after all, be the idtimate 

 mode ; there may be other links to be discovered between the sup- 

 posed cause and the effect; but We can only fiirther resolve the knoA\Ti 

 law, by introducing some other law hitherto unknowTi ; which will not 

 diminish the number of ultimate laws. 



In what cases, accordingly, has science been most successful in 

 explaining phenomena, by resolving their complex laws into laws of 

 greater simplicity and generality? Hitherto chiefly in cases of the 

 propagation of various phenomena through space : and, first and prin- 

 cipally, the most extensive and important of all facts of that description, 

 the fact of motion. Now this is entirely what might be expected from 

 the principles which I have laid doA\Ti. Not only is motion one of the 

 most universal of all phenomena, it is also (as might be expected 

 from the former ciixumstance) one of those which, apparently at least, 

 are produced in the greatest number of ways : but the phenomepon 

 itself is always, to our sensations, the same in every respect but degree. 

 Differences of duration, or of velocity, are e^ddently differences in 

 degree only ; and differences of direction in space, which alone has 

 any semblance of being a distinction in kind, entirely disappear (so far 

 as our sensations are concerned) by a change in our o^^'n position ; 

 indeed the very same motion appears to us, according to our position, 

 to take place in every variety of direction, and njotions in every 

 different direction to take place in the same. And, again, motion in' 

 a straight line and in a cur\-e are no otherwise distinct than that the 

 one is motion continuing in the same direction, the other is motion 

 which at each instant changes its direction. Thei-e 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 consisted in resolving one observed law of the production 

 of motion into the laws of other known modes of production, 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 produced by magnetism were sho'ATi to be 

 produced by electricity ; when the motions of fluids in a lateral direc- 

 tion, or even contrary to the direction of gravity, were shoAvm to be 

 produced by gravity ; and the like. There is an abundance of distinct 



