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The validity of the principle of relativity was assumed only for

these reference-bodies, but not for others (e.g. those possessing

motion of a different kind). In this sense we speak of the

special principle of relativity, or special theory of relativity.

In contrast to this we wish to understand by the "general

principle of relativity" the following statement: All bodies of

reference K, K′, etc., are equivalent for the description of

natural phenomena (formulation of the general laws of nature),

whatever may be their state of motion. But before proceeding

farther, it ought to be pointed out that this formulation must be

replaced later by a more abstract one, for reasons which will

become evident at a later stage.