QQ SCIENCE IN THE LAST HALF CENTURY. 



facts of nature, of iuductive aud deductive reasouiug for the discovery 

 of their mutual relations and connection. The various branches of 

 physical science difler in the extent to which, at any given moment of 

 thtir history, observation on the one hand, or ratiocination on the 

 other, is their more obvious feature, but in no other way ; and nothing 

 can be more incorrect than the assumption one sometimes meets witb, 

 that physics has one method, chemistry another, aud biology a third. 



POSTULATES. 



All physical science starts from certain postulates. One of them is 

 the objective existence of a material world. It is assumed that the 

 phenomena Avhich are comprehended under this name have a " sub- 

 stratum " of extended, impenetrable, mobile substance, which exhibits 

 the quality known as inertia, and is termed matter.* Another postu- 

 late is the universality of the law of causation : that nothing happens 

 without a cause (that is, a necessary precedent condition), and that the 

 state of the physical universe, at any given moment, is the consequence 

 of its state at any preceding moment. Another is that any of the rules, 

 or so-called "laws of nature," by which the relation of phenomena is 

 truly defined, is true for all time. The validity of these postulates is a 

 problem of metaphysics; they are neither self-evident nor are they, 

 strictly speaking, demonstrable. The justification of their employ- 

 ment, as axioms of physical i^hilosophy, lies in the circumstance that 

 expectations logically based upon them are verified, or, at any rate, 

 not contradicted, whenever they can be tested by experience. 



HYPOTHESES. 



Physical science therefore rests on verified or uncontradicted hypoth- 

 eses; and such being the case, it is not surprising that a great con- 

 dition of its progress has been the invention of verifiable hypotheses. 

 It is a favorite popular delusion that the scientific inquirer is under a 

 sort of moral obligation to abstain from going beyond that generaliza- 

 tion of observed facts which is absurdly called " Baconian " induction. 

 But any one who is practically acquainted with scientific work is aware 



" I am aw are that this proposition may be challenged. It may be said, for exam- 

 ple, that, on the hypothesis of Boscovich, matter has no extension, being reduced to 

 mathematical points serving as centers of "forces." But as the "forces'" of the 

 various centers are conceived to limit one another's action in such a manner Ihat 

 an area around each center has an individuality of its own, extension comes back in 

 the form of that ai'ea. Again, a very eminent mathematician aud physicist, the late 

 Clerk Maxwell, has declared that impenetrability is not essential to our notions of 

 matter, and that two atoms may conceivably occupy the same space. I am loth to 

 dispute any dictum of a philosopher as remarkable for the subtlety of his intellect as 

 for his vast knowledge ; but the assertion that one and the same point or area of 

 space can have different (conceivably opposite) attributes appears to me to violate 

 the principle of contradiction, which is the foundation not only of physical science, 

 but of logic m general. It means that A can be not-A. 



