HYPOTHESES 347 



and generality? Hitherto chiefly in cases of the propagation of various 

 phenomena through space ; and, first and principally, the most extensive 

 and important of all facts of that description, mechanical motion. Now 

 this is exactly what might be expected from the principles here laid down. 

 Not only is motion one of the most universal of all phenomena, it is also 

 (as might be expected from that circumstance) one of those which, appar- 

 ently at least, are produced in the greatest number of ways ; but the phe- 

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

 degree. Differences of duration or of velocity, are evidently 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 own position ; indeed, the 

 very same motion appears to us, according to our position, to take place in 

 every variety of direction, and motions in every different direction to take 

 place in the same. And again, motion in a straight line and in a curve ai'e 

 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. 

 There is, therefore, according to the principles 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 j^hys- 

 ical 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 shown to be produced by 

 electricity ; when the motions of fluids in a lateral direction, or even con- 

 trary 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 un- 

 resolved into one another : gravitation, heat, electricity, chemical action, 

 nervous action, and so forth ; but whether the efforts of the present gener- 

 ation of savants to resolve all these different modes of production into one 

 are ultimately successful or not, the attempt so to resolve them is perfect- 

 ly legitimate. For, though these various causes produce, in other respects, 

 sensations intrinsically 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 ; nor is it impossible that these various agencies 

 themselves may, as the new doctrines assert, all of them have for their own 

 immediate antecedent modes of molecular motion. 



We need not extend our illustration to other cases, as, for instance, to the 

 propagation of light, sound, heat, electricity, etc., through space, or any of 

 the other phenomena which have been found susceptible of explanation by 

 the resolution of their observed laws into more general laws. Enough has 

 been said to display the difference between the kind of explanation and 

 resolution of laws which is chimerical, and that of which the accomplish- 

 ment is the great aim of science; and to show into what sort of elements 

 the resolution must be effected, if at all.* 



* As is well remarked by Professor Bain, in the very valuable chapter of his Logic which 

 treats of this subject (ii., 121), "scientific explanation and inductive generalization being tlie 

 same thing, the limits of Explanation are the limits of Induction," and "the limits to induct- 

 ive generalization are the limits to the agreement or community of facts. Induction sup- 

 poses similarity among phenomena ; and when such similarity is discovered, it reduces the 



