520 



INDUCTION. 



ences 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 direc- 

 tion to take place in the same. And 

 again, motion in a straight line and 

 in a curve are no otherwise distinct 

 than that the one is motion continu- 

 ing in the same direction, the other 

 is motion which at each instant 

 changes its direction. There is, there- 

 fore, according to the principles I 

 have stated, no absurdity in suppos- 

 ing 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 mag- 

 netism 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 un- 

 resolved into one another — gravita- 

 tion, heat, electricity, chemical action, 

 nervous action, and so forth ; but 

 whether the efforts of the present 

 generation of savans to resolve all 

 these different modes of production 

 into one, are ultimately successful or 

 not, the attempt so to resolve them 

 is perfectly legitimate. For though 

 these various causes produce, in other 

 respects, sensations intrinsically dif- 

 ferent, and are not, therefore, capable 

 of beingf 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, elec- 

 tricity, &c., 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 be- 

 tween the kind of explanation and 

 resolution of laws which is chimeri- 

 cal, 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), " scien- 

 tific explanation and inductive generalisa- 

 tion being the same thing, the limits of 

 Explanation are the limits of Induction," 

 and "the limits to inductive generalisation 

 are the limits to the agreement or com- 

 munity of facts. Induction supposes simi- 

 larity among phenomena; and when such 

 stmilarity is discovered, it reduces the 

 phenomena under a common statement. 

 The similarity of terrestrial gravity to 

 celestial attraction enables the two to be 

 expressed as one phenomenon. The simi- 

 larity between capillary attraction, solu- 

 tion, the operation of cements, &c., leads 

 to their being regarded not as a plurality, 

 but as a unity, a single causative link, the 

 operation of a single agency. ... If it be 

 asked whether we can merge gravity itself 

 in some still higher law, the answer must 

 depend upon the facts. Are there any 

 other forces, at present held distinct from 

 gravity, that we may hope to make frater- 

 nise with it, so as to join in constituting a 

 higher unity? Gravity is an attractive 

 force ; and another great attractive force is 

 cohesion, or the force that binds together 

 the atoms of solid matter. Might we then 

 join these two in a still higher unity, ex- 

 pressed under a more comprehensive law ? 

 Certainly we might, but not to any advan- 

 tage. The two kinds of force agree in the 

 one point, attraction, but they agree in no 

 other : indeed, in the manner of the attrac- 



