106 PROCEEDINGS OF SECTION A. 
Mass we sometimes describe as “‘ quantity of matter.’’ We 
may find it possible to replace this by “ quantity of ether,” 
and to “explain” ordinary matter as a form of ether; but 
the three categories of space, time, and substance (whether 
matter or ether) we cannot further resolve. These are 
mental concepts—things derived from  self-consciousness. 
We cannot get away from ourselves, and struggle against 
it as we will, our science is doomed to be tinged with anthro- 
pomorphism. Starting with the three fundamentals— 
space, time, and mass—we want further a single “ law ”’ con- 
necting them, and one that will be comprehensive enough 
to group together ail the facts of the physical world. We 
cannot say that we have got this, or we should have solved 
the riddle of the physical universe, so far as science could 
ever solve it. As a connecting-link between the funda- 
mental ideas we might take the three laws of Newton. They 
are wide enough to satisfy our demands; but as we are in 
search of the greatest possible simplicity, we naturally try to 
reduce the number of these laws. We may replace them 
by two—the Law of Inertia, and Gauss’ Principle of Least 
Constraint—or we may combine these two into one, as Hertz 
has done in his Law of the Straightest Path; or, finally, we 
may use the Principle of Least Action. Our aim is to 
“explain ”’ everything, 7.c., to co-ordinate all our knowledge, 
by means of this one principle. There is still much to be 
done before the final victory is won. We have still to talk 
‘of chemical affinities, of cohesive forces, of the “law of 
gravity’; but we may be permitted to have faith that these 
and all other obscurities will be removed, and that some 
day the whole universe will be seen to be the expression of 
one great “law.” “Give me extension and motion,” 
exclaimed Descartes, ‘and JI will construct: the world!’ 
‘““Give me space, time, mass, and che fundamental law of 
motion,” says the modern physicist, “and I will construct 
the world, although I will not pretend to ‘explain’ it.” 
And it should be observed chat this method differs essenti- 
ally from what is sometimes called the metaphysical method. 
I am not going to say anything against metaphysics. It 
would be an impertinence to do so; and, as a rule, the 
remarks of scientists about metaphysics are (to say the least) 
quite as valueless as those of metaphysicians about science. 
All that need be said is that physicists do not evolve the 
world from their consciousness. Our knowledge is strictly 
empirical. Our hypotheses are valued only in so far as 
they fit together our experiences; everywhere they must be 
put to this test, and if they do not satisfy it, they are aban- 
