SIDELIGHTS ON RELATIVITY

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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