514 THE PRINCIPLES OF SCIENCE. [CHAP. 



Consistency with the Laws of Nature. 



In the second place an hypothesis must not be contra- 

 dictory to what we believe to be true concerning Nature. 

 It must not involve self-inconsistency which is opposed to 

 the highest and .simplest laws, namely, those of Logic. 

 Neither ought it to be irreconcilable with the simple 

 laws of motion, of gravity, of the conservation of energy, 

 nor any parts of physical science which we consider to be 

 established beyond reasonable doubt. Not that we are 

 absolutely forbidden to entertain such an hypothesis, but 

 if we do so we must be prepared to disprove some of the 

 best demonstrated truths in the possession of mankind. 

 The fact that conflict exists means that the consequences 

 of the theory are not verified if previous discoveries are 

 correct, and we must therefore show that previous dis- 

 coveries are incorrect before we can verify our theory. 



An hypothesis will be exceedingly improbable, not to 

 say absurd, if it supposes a substance to act in a manner 

 unknown in other cases ; for it then fails to be verified in 

 our knowledge of that substance. Several physicists, 

 especially Euler and Grove, have supposed that we might 

 dispense with an ethereal basis of light, and infer from 

 the interstellar passage of rays that there was a kind of 

 rare gas occupying space. But if so, that gas must be 

 excessively rare, as we may infer from the apparent 

 absence of an atmosphere around the moon, and from 

 other facts known to us concerning gases and the atmo- 

 sphere ; yet it must possess an elastic force at least a 

 billion times as great as atmospheric air at the earth's 

 surface, in order to account for the extreme rapidity of 

 light rays. Such an hypothesis then is inconsistent with 

 our knowledge concerning gases. 



Provided that there be no clear and absolute conflict 

 with known laws of nature, there is no hypothesis so 

 improbable or apparently inconceivable that it may not 

 be rendered probable, or even approximately certain, by 

 a sufficient number of concordances. In fact the two best 

 founded and most successful theories in physical science 

 involve the most absurd suppositions. Gravity is a force 

 irhich appears to act between bodies through vacuous 



