 When one attempts to write general relativity down in this way, one has to contend with the existence of constraints on the canonical variables that are inherited from the diffeomorphism invariance of the spacetime formulation of the theory. The single tensorial equation that we see in standard presentations of the Einstein field equations is translated into 10 scalar equations in the canonical formulation, with constraints accounting for four of these equations (the remaining six are genuine evolutionary equations). Three of the constraints (known as the momentum or diffeomorphism constraints) are responsible for shifting data tangential to the initial surface and, thus, are related to the shift vector field. The remaining constraint, known as the Hamiltonian (or scalar) constraint, is responsible for pushing data off the initial surface, and thus is related to the lapse function. If the constraints are not satisfied by the canonical initial data then the development of the data with respect to the evolution equations, will not generate a physically possible spacetime for choices of lapse and shift. However, when the constraints are satisfied then the various choices of lapse and shift will always grow the same 4D spacetime (that it, the same spacetime metric). However, to extract a notion of time from this formulation demands that one first solve for the spacetime metric, followed by a singling out of a specific solution. This is a kind of classical problem of time in that since the spacetime geometry is a dynamical variable, time is something that also must be solved for. Further, there is arbitrariness in the time variable as a result of the arbitrariness encoded in the constraints, stemming from the fact that time is essentially a freely chosen label of the three dimensional slices and so is not a physical parameter. However, one can extract a time for each solution to the Einstein equations by ‘deparametrizing’ the theory (i.e. isolating a variable from within the phase space that is to play the role of time). Below we see that things become more problematic in the shift to quantum theory.