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That which determines the magnitude of the tension of the rope is

the _inertial mass_ of the body." Guided by this example, we see

that our extension of the principle of relativity implies the

necessity of the law of the equality of inertial and

gravitational mass. Thus we have obtained a physical

interpretation of this law.

From our consideration of the accelerated chest we see that a

general theory of relativity must yield important results on the

laws of gravitation. In point of fact, the systematic pursuit of

the general idea of relativity has supplied the laws satisfied by

the gravitational field. Before proceeding farther, however, I

must warn the reader against a misconception suggested by these

considerations. A gravitational field exists for the man in the

chest, despite the fact that there was no such field for the

co-ordinate system first chosen. Now we might easily suppose that

the existence of a gravitational field is always only an apparent

one. We might also think that, regardless of the kind of

gravitational field which may be present, we could always choose

another reference-body such that no gravitational field exists

with reference to it. This is by no means true for all

gravitational fields, but only for those of quite special form.