  We briefly discuss the current status of Mach's principle in general
relativity and point out that its last vestige, namely, the gravitomagnetic
field associated with rotation, has recently been measured for the earth in the
GP-B experiment. Furthermore, in his analysis of the foundations of Newtonian
mechanics, Mach provided an operational definition for inertial mass and
pointed out that time and space are conceptually distinct from their
operational definitions by means of masses. Mach recognized that this
circumstance is due to the lack of any a priori connection between the inertial
mass of a body and its Newtonian state in space and time. One possible way to
improve upon this situation in classical physics is to associate mass with an
extra dimension. Indeed, Einstein's theory of gravitation can be locally
embedded in a Ricci-flat 5D manifold such that the 4D energy-momentum tensor
appears to originate from the existence of the extra dimension. An outline of
such a 5D Machian extension of Einstein's general relativity is presented.
