SIDELIGHTS ON RELATIVITY

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is the equivalence of the practically-rigid body and the body of
geometry--which suggests itself so readily--denied by Poincare and
other investigators? Simply because under closer inspection the
real solid bodies in nature are not rigid, because their geometrical
behaviour, that is, their possibilities of relative disposition,
depend upon temperature, external forces, etc. Thus the original,
immediate relation between geometry and physical reality appears
destroyed, and we feel impelled toward the following more general
view, which characterizes Poincare's standpoint. Geometry (G)
predicates nothing about the relations of real things, but only
geometry together with the purport (P) of physical laws can do so.
Using symbols, we may say that only the sum of (G) + (P) is subject
to the control of experience. Thus (G) may be chosen arbitrarily,
and also parts of (P); all these laws are conventions. All that
is necessary to avoid contradictions is to choose the remainder of
(P) so that (G) and the whole of (P) are together in accord with
experience. Envisaged in this way, axiomatic geometry and the part