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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 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.