The doctrine of the relativity of motion is attractive for its simplicity.
According to it, the assertion that a body moves can mean nothing more
than that it moves with respect to other bodies. Acceleration has long
proved to be the stumbling block for the doctrine, for, in the case of
acceleration, the simplest of observations seem to contradict the doctrine.
When a test body rotates, for example, it is acted upon by centrifugal
forces. The presence of these centrifugal forces seems to be completely
independent of whether the test body rotates with respect to bodies
immediately surrounding. Thus Newton observed in his famous bucket
experiment that these centrifugal forces induced a concavity in the
surface of a rotating body of water and did so independently of whether
the water rotated with respect to the bucket containing the water.
Therefore, using these inertial forces as a marker to indicate whether the
body is accelerating, it seems possible to know that a body is
accelerating without any concern for whether it accelerates with respect
to the other bodies around it. This outcome contradicts the doctrine of
the relativity of motion as applied to acceleration.

The proposal’s most prominent sponsor was Albert Einstein. In the
early years of his work on general relativity, he believed that his theory
implemented the proposal, although he completely lost this belief in his
later years. Nonetheless, the future of the proposal was guaranteed by the
vigorous support of an Einstein who rapidly rose to celebrity status both
inside and outside the scientific community. Einstein did not claim the
proposal as his own invention. From the earliest moments, he attributed
it to Ernst Mach and in 1918 gave a field theoretic formulation of the
proposal its now standard name of “Mach’s Principle.’