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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.