Therefore, the two step process in the classical theory of solving the constraints formula_83 (equivalent to solving the admissibility conditions for the initial data) and looking for the gauge orbits (solving the 'evolution' equations) is replaced by a one step process in the quantum theory, namely looking for solutions formula_79 of the quantum equations formula_85. This is because it obviously solves the constraint at the quantum level and it simultaneously looks for states that are gauge invariant because formula_86 is the quantum generator of gauge transformations (gauge invariant functions are constant along the gauge orbits and thus characterize them). Recall that, at the classical level, solving the admissibility conditions and evolution equations was equivalent to solving all of Einstein's field equations, this underlines the central role of the quantum constraint equations in canonical quantum gravity.