 The preferred approach in canonical quantum gravity is to impose the constraints after quantizing. In this ‘constraint quantization’ approach, due to Dirac, one treats the constraints themselves as operators A, and demands that “physical” states ψ be those which are solutions to the resulting equations A ψ = 0. The problem of time is associated with the super-Hamiltonian constraint, as mentioned above. The super-Hamiltonian H is responsible for describing time-evolution in the classical theory, yet its counterpart in the constraint-quantized theory, H ψ = 0, would prima facie seem to indicate that the true physical states of the system do not evolve at all: there is no t. Trying to understand how, and in what sense, the quantum theory describes the time-evolution of something, be it states or observables, is the essence of the problem of time (on which, more below).