OF NATURAL PHILOSOPHY. 197 



some mode of expression common to both, at least 

 in so far as the phenomena actually known are 

 concerned. Now, this is a very great step, not 

 only for its own sake, as leading us to a high point 

 in philosophical speculation, but for its applica- 

 tions; because whatever conclusions we deduce from 

 an hypothesis so supported must have at least a 

 strong presumption in their favour: and we may 

 be thus led to the trial of many curious experi- 

 ments, and to the imagining of many useful and 

 important contrivances, which we should never 

 otherwise have thought of, and which, at all events, 

 ^ verified in practice, are real additions to our stock 

 of knowledge and to the arts of life. 



(209.) In framing a theory which shall render 

 a rational account of any natural phenomenon, we 

 have first to consider the agents on which it de- 

 pends, or the causes to which we regard it as 

 ultimately referable. These agents are not to be 

 arbitrarily assumed ; they must be such as we have 

 good inductive grounds to believe do exist in nature, 

 and do perform a part in phenomena analogous to 

 those we would render an account of; or such, 

 whose presence in the actual case can be demon- 

 strated by unequivocal signs. They must be verfK- 

 causce, in short, which we can not only show to exist 

 and to act, but the laws of whose action we can de- 

 rive independently, by direct induction, from expe- 

 riments purposely instituted ; or at least make such 

 suppositions respecting them as shall not be contrary 

 to our experience, and which will remain to be 

 verified by the coincidence of the conclusions we 

 shall deduce from them, with facts. For example, in 

 o 3 



