140 THE PRINCIPLES OF SCIENCE. 



(3) That the consequences inferred do agree with facts 

 of observation. 



The First Requisite Possibility of Deductive 

 Reasoning. 



As the truth of an hypothesis is to be proved by its con- 

 formity with fact, the first condition is that we be able 

 to apply methods of deductive reasoning, and learn what 

 should happen according to such an hypothesis. Even if 

 we could imagine an object acting according to laws 

 wholly unknown in other parts of nature, it would be 

 useless to do so, because we could never decide whether it 

 existed or riot. We can only infer what would happen 

 under supposed conditions by applying what knowledge 

 we possess of nature to those conditions. Hence, as Bos- 

 covich truly said, we are to understand by hypotheses 

 'not fictions altogether arbitrary, but suppositions con- 

 formable to experience or analogy/ It follows that every 

 hypothesis" worthy of consideration must suggest some 

 likeness, analogy, or common law, acting in two or more 

 things. If, in order to explain certain facts, a, a', a", &c., 

 we invent a cause A, then we must in some degree appeal 

 to experience as to the mode in which A will act. As the 

 objects and laws of nature are certainly not known to the 

 mind intuitively, we must point out some other cause B, 

 which supplies the requisite notions, and all we do/is .to 

 invent a fourth term to an analogy. As B is to its effects 

 Z>, V, I", &c., so is A to its effects a, a', a", &c. When, for 

 instance, we attempt to explain the passage of light and 

 heat radiations through space unoccupied by matter, we 

 imagine the existence of the so-called ether. But if this 

 ether were wholly different from anything else known to 

 us, we should in vain try to reason about_iL__Je must 

 at least apply to it the laws of motion, that is, we must 



