THE DEDUCTIVE METHOD. 



305 



Composition of Causes, since (except 

 in a few cases not of primary im- 

 portance) each of the heavenly bodies 

 may be considered, without material 

 inaccuracy, to be never at one time 

 influenced by the attraction of more 

 than two bodies, the sun and one 

 other planet or satellite ; making with 

 the reaction of the body itself, and 

 the force generated by the body's own 

 motion and acting in the direction of 

 the tangent, only four different agents 

 on the concurrence of which the mo- 

 tions of that body depend ; a much 

 smaller number, no doubt, than that 

 by which any other of the great phe- 

 nomena of nature is determined or 

 modified. Yet how could we ever 

 have ascertained the combination of 

 forces on which the motions of the 

 earth and planets are dependent by 

 merely comparing the orbits or veloci- 

 ties of different planets, or the dif- 

 ferent velocities or positions of the 

 same planet? Notwithstanding the 

 regularity which manifests itself in 

 those motions, in a degree so rare 

 among the effects of concurrence of 

 causes ; and although the periodical 

 recurrence of exactly the same effect 

 affords positive proof that all the 

 combinations of causes which occur 

 at all, recur periodically ; we should 

 not have known what the causes were, 

 if the existence of agencies precisely 

 similar on our own earth had not, 

 fortunately, brought the causes them- 

 selves within the reach of experimen- 

 tation under simple circumstances. 

 As we shall have occasion to analyse, 

 farther on, this great example of the 

 Method of Deduction, we shall not 

 occupy any time with it here, but 

 shall proceed to that secondary appli- 

 cation of the Deductive Method the 

 result of which is not to prove laws of 

 phenomena, but to explain them. 



CHAPTER XII. 



OF THE EXPLANATION OF LAW3 OF 

 NATURE. 



§ I. The deductive operation by 

 which we derive the law of an effect 

 from the laws of the causes, the con- 

 currence of which gives rise to it, 

 may be undertaken either for the 

 purpose of discovering the law, or of 

 explaining a law already discovered. 

 The word explanation occurs so con- 

 tinually and holds so important a 

 place in philosophy, that a little time 

 spent in fixing the meaning of it will 

 be profitably employed. 



An individual fact is said to be 

 explained by pointing out its cause, 

 that is, by stating the law or laws of 

 causation of which its production is 

 an instance. Thus a conflagration is 

 explained when it is proved to have 

 arisen from a spark falling into the 

 midst of a heap of combustibles ; and 

 in a similar manner, a law of uni- 

 formity in nature is said to be ex- 

 plained when another law or laws 

 are pointed out, of which that law 

 itself is but a case, and from which it 

 could be deduced. 



§ 2. There are three distinguish- 

 able sets of circumstances in which a 

 law of causation may be explained 

 from, or, as it also is often expressed, 

 resolved into, other laws. 



The first is the case already so 

 fully considered ; an intermixture of 

 laws, producing a joint effect equal 

 to the sum of the effects of the causes 

 taken separately. The law of the 

 complex effect is explained by being 

 resolved into the separate laws of the 

 causes which contribute to it. Thus 

 the law of the motion of a planet is 

 resolved into the law of the acquired 

 force which tends to produce an uni- 

 form motion in the tangent, and the 

 law of the centripetal force which 

 tends to produce an accelerating mo- 

 tion towards the sun ; the real motion 

 being a compound of the two. 



It is necessary here to remark, that 



U 



