 As noted in  Section 3.2.2  above, the treatment of time presents special difficulties in canonical quantum gravity, though they easily generalise to many other approaches to quantum gravity. These difficulties are connected with the special role time plays in physics, and in quantum theory in particular. Physical laws are, in general, laws of motion, of change from one time to another. They represent change in the form of differential equations for the evolution of, as the case may be, classical or quantum states; the state represents the way the system is at some time, and the laws allow one to predict how it will be in the future (or retrodict how it was in the past). It is not surprising, then, that a theory of quantum spacetime would have a problem of time, because there is no classical time against which to evolve the “state”. The problem is not so much that the spacetime is dynamical; there is no problem of time in classical general relativity (in the sense that a time variable is present). Rather, the problem is roughly that in quantizing the structure of spacetime itself, the notion of a quantum state, representing the structure of spacetime at some instant, and the notion of the evolution of the state, do not get any traction, since there are no real “instants”. (In some approaches to canonical gravity, one fixes a time before quantizing, and quantizes the spatial portions of the metric only. This approach is not without its problems, however; see Isham (1993) for discussion and further references.)