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We must note carefully that the possibility of this mode of

interpretation rests on the fundamental property of the

gravitational field of giving all bodies the same acceleration,

or, what comes to the same thing, on the law of the equality of

inertial and gravitational mass. If this natural law did not

exist, the man in the accelerated chest would not be able to

interpret the behaviour of the bodies around him on the

supposition of a gravitational field, and he would not be

justified on the grounds of experience in supposing his

reference-body to be "at rest."