  The task of quantizing gravity is compared with Einstein's relativization of
gravity. The philosophical and physical foundations of general relativity are
briefly reviewed. The Ehlers-Pirani-Schild scheme of operationally determining
the geometry of space-time, using freely falling classical particle
trajectories, is done using operations in an infinitesimal neighborhood around
each point. The study of the free fall of a quantum wave suggests a quantum
principle of equivalence. The principle of general covariance is clarified. The
sign change of a Fermion field when rotated by $2\pi$ radians is used to argue
for a quantum mechanical modification of space-time, which leads naturally to
supersymmetry. A novel effect in quantum gravity due to the author is used to
extend Einstein's hole argument to quantum gravity. This suggests a quantum
principle of general covariance, according to which the fundamental laws of
physics should be covariant under `quantum diffeomorphisms'. This heuristic
principle implies that space-time points have no invariant meaning in quantum
gravity.
