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The principle we have made use of not only maintains that we may

equally well choose the carriage or the embankment as our

reference-body for the description of any event (for this, too,

is self-evident). Our principle rather asserts what follows: If

we formulate the general laws of nature as they are obtained from

experience, by making use of

(a) the embankment as reference-body,

(b) the railway carriage as reference-body,