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is tin- universality of the law of causation; 

 that nothing happens without a cause (that is, a 



ssary precedent condition), and that the state 

 of the physical universe, at any given moment, is 

 the consequence of its state at any preceding 

 nioi in-lit. Another is that any of the rules, or 



ilh-tl " 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 



i physics ; they are neither self-evident nor 

 are they, strictly speaking, demonstrable. The 

 justification of their employment, as axioms of 

 physical philosophy, lies in the circumstance that 



tat inns logically baaed upon them are verified, 

 or. al any rate, not contradicted, whenever they 



1 by experience. 



Physical science therefore rests on verified or 

 uncontradicted hypotheses; and, such being the 

 case, it is not surprising that a great condition of 



may be said, for example, that, on the hypothesis of Boscovich, 



i r has no extrusion, teing reduced to mathematical points 



servingas centres of " forces." But as the " forces" of the various 



; --s are conceived to limit one another's action in such a 



manner that an area around each centre has an individuality of 



\tension comes back in the form of that area. Again, 



.: nrnt mathematician and physicist the late Clerk 



has declared that impenetrability is not essential to 



our in it i< ins of matter, and that two atoms may conceivably 



y the same space. I am loth to dispute any dictum of a 



philosopher as remarkable for the subtlety of his intellect as for 



ist knowledge ; but the assertion that one and the same 

 ; or area of space can have different (conceivably opposite) 



attributes a]>]*-ars to me to violate the principle of contradic- 

 tion, which is the foundation not only of physical science, but 

 of logic in general. It means that A can be not-A, 



