It was at all times clear that, from the point of view of the idea it
conveys to us, every motion must be considered only as a relative
motion. Returning to the illustration we have frequently used of the
embankment and the railway carriage, we can express the fact of the
motion here taking place in the following two forms, both of which are
equally justifiable:
(_a_) The carriage is in motion relative to the embankment,
(_b_) The embankment is in motion relative to the carriage.
In (_a_) the embankment, in (_b_) the carriage, serves as the body of
reference in our statement of the motion taking place. If it is simply
a question of detecting or of describing the motion involved, it is in
principle immaterial to what reference-body we refer the motion. As
already mentioned, this is self-evident, but it must not be confused
with the much more comprehensive statement called the principle of
relativity, which we have taken as the basis of our investigations.
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,
then these general laws of nature (_e.g._ the laws of mechanics or the
law of the propagation of light _in vacuo_) have exactly the same form
in both cases. This can also be expressed as follows: For the physical
description of natural processes, neither of the reference bodies _K,
K _ is unique (lit. specially marked out ) as compared with the other.
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