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
of natural law which has been given a conventional status appear
as epistemologically equivalent.

_Sub specie aeterni_ Poincare, in my opinion, is right. The idea
of the measuring-rod and the idea of the clock co-ordinated with it
in the theory of relativity do not find their exact correspondence
in the real world. It is also clear that the solid body and the
clock do not in the conceptual edifice of physics play the part of
irreducible elements, but that of composite structures, which may
not play any independent part in theoretical physics. But it is my
conviction that in the present stage of development of theoretical
physics these ideas must still be employed as independent ideas;
for we are still far from possessing such certain knowledge
of theoretical principles as to be able to give exact theoretical
constructions of solid bodies and clocks.

Further, as to the objection that there are no really rigid bodies
in nature, and that therefore the properties predicated of rigid
bodies do not apply to physical reality,--this objection is by