OF NATURAL PHILOSOPHY. 119 



bearing on the point in question. Common sense 

 dictates this, as affording us the means of examining 

 the same subject in several points of view ; and it 

 would also dictate, that the more different these 

 collected facts are in all other circumstances but 

 that which forms the subject of enquiry, the better; 

 because they are then in some sort brought into 

 contrast with one another in their points of dis- 

 agreement, and thus tend to render those in which 

 they agree more prominent and striking. 



(110.) The only facts which can ever become useful 

 as grounds of physical enquiry are those which 

 happen uniformly and invariably under the same 

 circumstances. This is evident: for if they have 

 not this character they cannot be included in laws ; 

 they want that universality which fits them to enter 

 as elementary particles into the constitution of those 

 universal axioms, which we aim at discovering. If one 

 and the same result does not constantly happen under 

 a given combination of circumstances, apparently the 

 same, one of two things must be supposed, caprice 

 (i.e. the arbitrary intervention of mental agency), or 

 differences in the circumstances themselves, really 

 existing, but unobserved by us. In either case, 

 though we may record such facts as curiosities, or 

 as awaiting explanation when the difference of cir- 

 cumstances shall be understood, we can make no 

 use of them in scientific enquiry. Hence, when- 

 ever we notice a remarkable effect of any kind, our 

 first question ought to be, Can it be reproduced ? 

 What are the circumstances under which it has 

 happened ? And will it always happen again if those 

 I 4 



