38 SCIENTIFIC INVESTIGATION 



comprehended under this name have a "substratum" 

 of extended, impenetrable, mobile substance, which ex- 

 hibits the quality known as inertia, and is termed 

 matter. 2 Another postulate 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 

 denned, is true for all time. The validity of these pos- 

 tulates is a problem of metaphysics; 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 ex- 

 pectations logically based upon them are verified, or, at 

 any rate, not contradicted, whenever they can be tested 

 by experience. 



Physical science therefore rests on verified or uncontra- 

 dicted hypotheses ; and, such being the case, it is not sur- 

 prising that a great condition of its progress has been the 

 invention of verifiable hypotheses. It is a favourite popu- 



2 I am aware that this proposition may be challenged. It 

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

 matter has no extension, being reduced to mathematical points 

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

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

 manner that an area around each centre has an individuality of 

 its own, extension comes back in the form of that area. Again, 

 a very eminent mathematician and 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 

 in general. It means that A can be not-A. [T. H. H.]. 



