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We now proceed to the second argument, to which, moreover, we

shall return later. If the principle of relativity (in the

restricted sense) does not hold, then the Galileian co-ordinate

systems K, K′, K″, etc., which are moving uniformly relative to

each other, will not be equivalent for the description of natural

phenomena. In this case we should be constrained to believe that

natural laws are capable of being formulated in a particularly

simple manner, and of course only on condition that, from amongst

all possible Galileian co-ordinate systems, we should have chosen

one (K0) of a particular state of motion as our body of

reference. We should then be justified (because of its merits for

the description of natural phenomena) in calling this system

"absolutely at rest," and all other Galileian systems K "in

motion." If, for instance, our embankment were the system K0 then

our railway carriage would be a system K, relative to which less

simple laws would hold than with respect to K0.