In the monograph (GDR, 1971) prof. Treder, a well-known German scientist dealing with the problems of gravity, is given a thorough analysis of the principle of equivalence, which Einstein put into the basis of the theory of gravity he created - the general theory of relativity. The axiomatics of the general theory of relativity has not yet been created, but the author's research is very relevant. This is a monograph on one of the fundamental questions of modern theory of gravity.