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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.