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It is true that this important law had hitherto been recorded in

mechanics, but it had not been interpreted. A satisfactory

interpretation can be obtained only if we recognise the following

fact: The same quality of a body manifests itself according to

circumstances as "inertia" or as "weight" (lit. "heaviness"). In

the following section we shall show to what extent this is

actually the case, and how this question is connected with the

general postulate of relativity.

XX.

THE EQUALITY OF INERTIAL AND GRAVITATIONAL MASS AS AN ARGUMENT

FOR THE GENERAL POSTULATE OF RELATIVITY