 The expression on the right side, introduced by a coupling term that contains Newton’s gravitational constant, mathematically represents the non-gravitational sources of the gravitational field in a region of spacetime in the form of a stress-energy-momentum tensor (an “omnium gatherum” in Eddington’s (1919: 63) pithy phrase). As the geometry of spacetime principally resides on the left-hand side, this situation seems unsatisfactory. Late in life, Einstein likened his famous equation to a building, one wing of which (the left) was built of “fine marble”, the other (the right) of “low grade wood” (1936: 311). In its classical form, general relativity accords only the gravitational field a direct geometrical significance; the other physical fields reside in spacetime; they are not of spacetime.