  Einstein regarded as one of the triumphs of his 1915 theory of gravity ---
the general theory of relativity --- that it vindicated the action--reaction
principle, while Newtonian mechanics as well as his 1905 special theory of
relativity supposedly violated it. In this paper we examine why Einstein came
to emphasise this position several years after the development of general
relativity. Several key considerations are relevant to the story: the
connection Einstein originally saw between Mach's analysis of inertia and both
the equivalence principle and the principle of general covariance, the waning
of Mach's influence owing to de Sitter's 1917 results, and Einstein's detailed
correspondence with Moritz Schlick in 1920.
