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space co-ordinates as opposed to the time co-ordinate. Every

point on the mollusc is treated as a space-point, and every

material point which is at rest relatively to it as at rest, so

long as the mollusc is considered as reference-body. The general

principle of relativity requires that all these molluscs can be

used as reference-bodies with equal right and equal success in

the formulation of the general laws of nature; the laws

themselves must be quite independent of the choice of mollusc.

The great power possessed by the general principle of relativity

lies in the comprehensive limitation which is imposed on the laws

of nature in consequence of what we have seen above.

XXIX.

THE SOLUTION OF THE PROBLEM OF GRAVITATION ON THE BASIS OF THE

GENERAL PRINCIPLE OF RELATIVITY

If the reader has followed all our previous considerations, he

will have no further difficulty in understanding the methods

leading to the solution of the problem of gravitation.