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body” and derivable from them; only experience can decide as to its correctness or incorrectness. Up to the present, however, we have by no means maintained the equivalence of all bodies of reference $K$ in connection with the formulation of natural laws. Our course was more on the following lines. In the first place, we started out from the assumption that there exists a reference-body $K$, whose condition of motion is such that the Galileian law holds with respect to it: A particle left to itself and sufficiently far removed from all other particles moves uniformly in a straight line. With reference to $K$ (Galileian reference-body) the laws of nature were to be as simple as possible. But in addition to $K$, all bodies of reference $K'$ should be given preference in this sense, and they should be exactly equivalent to $K$ for the formulation of natural laws, provided that they are in a state of uniform rectilinear and non-rotary motion with respect to $K$; all these bodies of reference are to be regarded as Galileian reference-bodies. 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