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But if rods of every kind (i.e. of every material) were to behave

_in the same way_ as regards the influence of temperature when

they are on the variably heated marble slab, and if we had no

other means of detecting the effect of temperature than the

geometrical behaviour of our rods in experiments analogous to the

one described above, then our best plan would be to assign the

distance one to two points on the slab, provided that the ends of

one of our rods could be made to coincide with these two points;

for how else should we define the distance without our proceeding

being in the highest measure grossly arbitrary? The method of

Cartesian coordinates must then be discarded, and replaced by

another which does not assume the validity of Euclidean geometry

for rigid bodies.[20] The reader will notice that the situation

depicted here corresponds to the one brought about by the general

postulate of relativity (Section XXIII).