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phenomena in our neighbourhood, this question has no importance. 
In our neighbourhood the gz, have in all cases within the limits of 
accuracy of our observations the values (1), and the field-equations 
(4) are not different from (8). The question thus really is: how are 
we to extrapolate outside our neighbourhood? The choice can thus 
not be decided by physical arguments, but must depend on meta- 
physical or philosophical considerations, in which of course also 
personal judgment or predilections will have some influence. 
To the question: If all matter is supposed not to exist, with the 
exception of one material point which is to be used as a test-body, 
has then this test-body inertia or not? the school of Macr requires 
the answer No. Our experience however very decidedly gives the 
answer Ves, if by ‘all matter” is meant all ordinary physical matter : 
stars, nebulae, clusters, etc. The followers of Macu are thus com- 
pelled to assume the existence of still more matter : the world-matter. 
If we place ourselves on this point of view, we must .necessarily 
adopt the system A, which is the only one that admits a world- 
matter. *) 
This world-matter, however, serves no other purpose than to 
enable us to suppose it not to exist. Now the formula (6) shows, 
that if it does not exist (9 = 0), the field-equations are not satisfied : 
supposing it not to exist thus appears to be a logical impossi- 
bility; in the system A, the world-matter ís the three-dimensional 
space, or at least is inseparable from it. 2 
We can also abandon the postulate of Macu, and replace it by 
the postulate that at infinity the g,,, or only the gj; of three- 
dimensional space, shall be zero, or at least invariant for all trans- 
formations. This postulate can also be enounced by saying that it 
must be possible for the whole universe to perform arbitrary motions, 
which can never be detected by any observation. The three-dimen- 
sional world must, in order to be able to perform ‘‘motions”’, i.e. in 
order that its position can be a variable function of the time, be 
thought movable in an “absolute” space of three or more dimensions 
(not the time-space a, y, 2, ct). The four-dimensional world requires 
for its “motion” a four- (or more-) dimensional absolute space, and 
-moreover an extra-mundane “time” which serves as independent 
variable for this motion. All this shows that the postulate of the 
1) The hypothesis formerly held by Einstein, and denied by me, that it would 
be possible, with the equations (3) and by means of very large masses at very 
large distances, to get values of gu which would degenerate to an invariant set 
at infinity, has now been shown to be untenable by Ernsrein himself (lc. page 
146). 
