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Since the introduction of the special principle of relativity has

been justified, every intellect which strives after

generalisation must feel the temptation to venture the step

towards the general principle of relativity. But a simple and

apparently quite reliable consideration seems to suggest that,

for the present at any rate, there is little hope of success in

such an attempt; Let us imagine ourselves transferred to our old

friend the railway carriage, which is travelling at a uniform

rate. As long as it is moving uniformly, the occupant of the

carriage is not sensible of its motion, and it is for this reason

that he can without reluctance interpret the facts of the case as

indicating that the carriage is at rest, but the embankment in

motion. Moreover, according to the special principle of

relativity, this interpretation is quite justified also from a

physical point of view.