XXI
IN WHAT RESPECTS ARE THE FOUNDATIONS OF CLASSICAL MECHANICS AND OF THE SPECIAL THEORY OF RELATIVITY UNSATISFACTORY?
WE 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 which possess certain unique states of motion, and which are in uniform translational motion relative to each other. Relative to other reference-bodies the law is not valid. Both in classical mechanics and in the special theory of relativity we therefore differentiate between reference-bodies relative to which the recognised “laws of nature” can be said to hold, and reference-bodies 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-
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