32 THE ADVANCE OF SCIENCE 



mobile substance, which exhibits the qual- 

 ity known as inertia, and is termed mat- 

 ter.* Another postulate is the univer- 

 sality of the law of causation ; that noth- 

 ing 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 



* I am aware that this proposition may be chal- 

 lenged. It may be said, for example, that, on the hy- 

 pothesis 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 indi- 

 viduality 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 in- 

 tellect as for his vast knowledge; but the assertion 

 that one and the same point or aroa 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. 



