 A similar confusion can be found in discussions of the related problem of defining observables in canonical general relativity. The claim gets its traction from the fact that it is very difficult to construct observables in canonical general relativity, while (apparently) it is relatively straightforward in the standard Lagrangian description. (See, e.g., Curiel, 2009, pp. 59–60, for an explicit statement of this claim. Curiel cites a theorem of Torre, 1993, to the effect that there can be no local observables in compact spacetimes, to argue that the canonical formulation is defective somehow.) Again, this rests on a misunderstanding over what the canonical formalism is and how it is related to the standard spacetime formulation of general relativity. That there are no local observables is not an artefact of canonical general relativity. The notion that observables have to be non-local (in this case, relational) is a generic feature that results precisely from the full spacetime diffeomorphism invariance of general relativity (and is, in fact, implicit in the theorem of Torre mentioned earlier). It receives a particularly transparent description in the context of the canonical approach because one can define observables as quantities that commute with all of the constraints. The same condition will hold for the four-dimensional versions, only they will have to be spacetime diffeomorphism invariant in that case. This will still rule out local observables since any quantities defined at points or regions of the spacetime manifold will clearly fail to be diffeomorphism invariant. Hence, the problems of observables (and the result that they must be either global or relational in general relativity) is not a special feature of the canonical formulation, but a generic feature of theories possessing diffeomorphism invariance. As Ashtekar and Geroch point out, “[s]ince time is essentially a geometrical concept [in general relativity], its definition must be in terms of the metric. But the metric is also the dynamical variable, so the flow of time becomes intertwined with the flow of the dynamics of the system” (1974, p. 1215).