 While such an approach sounds sensible enough on the surface, to actually put it into practice in the constructive stages of theory-building (rather than a retrospective analysis of a completed theory) is not going to be easy—though it has to be said, the method Cao describes bears close resemblance to the way loop quantum gravity has developed. Lucien Hardy (2007) has developed a novel approach to quantum gravity that shares features of Cao’s suggestion, though the principles isolated are different from Cao’s. The causaloid approach is intended to provide a framework for quantum gravity theories, where idea is to develop a general formalism that respects the key features of both general relativity, which he takes to be the dynamical (non-probabilistic) causal structure, and quantum theory, which he takes to be the probabilistic (nondynamical) dynamics. The causaloid (of some theory) is an entity that encodes all that can be calculated in the theory. Part of the problem here is that Cao’s (and Hardy’s) approach assumes that the ontological principles hold at the Planck scale. However, it is perfectly possible that both of the input theories break down at higher energies. Not only that, the technical difficulties of setting up the kind of (physically realistic) diffeomorphism-invariant quantum field theory he suggests have so far proven to be an insurmountable challenge. One crucial aspect that is missing from Cao’s framework is a notion of what the observables might be. Of course, they must be relational, but this still leaves the problem very much open. (The idea of making progress by isolating appropriate principles of quantum gravity forms the basis of a special issue: Crowther and Rickles, eds, 2014.)