ABSTRACT
The general theory of relativity, Einstein's premier contribution to physics, displaced Newton's law of gravity, for two hundred years the acme of classical mechanics and epitome of an exact law of nature. To complete the general theory of relativity, Einstein would have to come to a new understanding of the requirement of general covariance. The general theory of relativity can be illuminated by reference to the three principles Einstein in 1918 considered undergirding the theory. These are the principle of equivalence, the principle of general relativity, and Mach's principle. Only formulated as a principle in 1912, the principle of equivalence is commonly illustrated by the "elevator" thought experiment used by Einstein, initially in a lecture in Vienna in December 1913. As Einstein also erroneously thought, the principle of general covariance was required by a principle of general relativity, i.e., the extension of the principle of relativity to accelerating frames of reference, this came as a heavy blow.
