332 INDUCTION. 



CHAPTER XII. 



OF THE EXPLANATION OF LAWS OF NATURE. 



§ 1. The deductive operation by which we derive the law of an effect 

 from the laws of the causes, the concurrence of which gives rise to it, may 

 be undertaken either for the purpose of discovering the law, or of explain- 

 ing a law already discovered. The word explanation occurs so continual- 

 ly, 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 or uniformity in nature is said to be explained, 

 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 distinguishable sets of circumstances in which a law 

 of causation may be explained from, or, as it also is often expressed, re- 

 solved 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 a uniform motion in the tangent, and the law of 

 the centripetal force, which tends to produce an accelerating motion toward 

 the sun ; the real motion being a compound of the two. 



It is necessary here to remark, that in this resolution of the law of a 

 complex effect, the laws of which it is compounded are not the only ele- 

 ments. It is resolved into the laws of the separate causes, together with 

 the. fact of their co-existence. The one is as essential an ingredient as the 

 other ; whether the object be to discover the law of the effect, or only to 

 explain it. To deduce the laws of the heavenly motions, we require not 

 only to know the law of a rectilineal and that of a gravitative force, but the 

 existence of both these forces in the celestial regions, and even their rela- 

 tive amount. The complex laws of causation are thus resolved into two 

 distinct kinds of elements : the one, simpler laws of causation, the other 

 (in the aptly selected expi-ession of Dr. Chalmers) collocations ; the colloca- 

 tions consisting in the existence of certain agents or powers, in certain 

 circumstances of place and time. We shall hereafter have occasion to re- 

 turn to this distinction, and to dwell on it at such length as dispenses with 

 the necessity of further insisting on it here. The first mode, then, of the 

 explanation of Laws of Causation, is when the law of an effect is resolved 

 into the various tendencies of which it is the result, together with the laws 

 of those tendencies. 



§ 3. A second case is when, between what seemed the cause and what 



