Page:Relativity (1931).djvu/104



E have already stated several times that classical mechanics starts out from the following law: Material particles sufficiently far removed from other material particles continue to move uniformly in a straight line or continue in a state of rest. We have also repeatedly emphasised that this fundamental law can only be valid for bodies of reference $$K$$ which possess certain unique states of motion, and which are in uniform translational motion relative to each other. Relative to other reference-bodies $$K$$ the law is not valid. Both in classical mechanics and in the special theory of relativity we therefore differentiate between reference-bodies $$K$$ relative to which the recognised “laws of nature” can be said to hold, and reference-bodies $$K$$ relative to which these laws do not hold.

But no person whose mode of thought is logical can rest satisfied with this condition of things. He asks: ‘‘How does it come that certain refer-