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. Unlike the first, this latter statement need not of necessity hold _a priori;_ it is not contained in the conceptions of “motion” and “reference-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 Iines. 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 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.