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The considerations of Section XX show that the general principle

of relativity puts us in a position to derive properties of the

gravitational field in a purely theoretical manner. Let us

suppose, for instance, that we know the space-time "course" for

any natural process whatsoever, as regards the manner in which it

takes place in the Galileian domain relative to a Galileian body

of reference K. By means of purely theoretical operations (i.e.

simply by calculation) we are then able to find how this known

natural process appears, as seen from a reference-body K′ which

is accelerated relatively to K. But since a gravitational field

exists with respect to this new body of reference K′, our

consideration also teaches us how the gravitational field

influences the process studied.