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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.