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.
THE EQUALITY OF INERTIAL AND GRAVITATIONAL MASS
AS AN ARGUMENT FOR THE GENERAL POSTULE OF RELATIVITY
We imagine a large portion of empty space, so far removed from stars
and other appreciable masses, that we have before us approximately the
conditions required by the fundamental law of Galilei. It is then
possible to choose a Galileian reference-body for this part of space
(world), relative to which points at rest remain at rest and points in
motion continue permanently in uniform rectilinear motion. As
reference-body let us imagine a spacious chest resembling a room with
an observer inside who is equipped with apparatus. Gravitation
naturally does not exist for this observer. He must fasten himself
with strings to the floor, otherwise the slightest impact against the
floor will cause him to rise slowly towards the ceiling of the room.
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