CHAPTER XII. 



OF THE EXPLANATION OP 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 explaining a law already 

 discovered. The word explanation occurs so continually 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, 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 uniform 

 motion in the tangent, and the law of the centripetal force 



