 Canonical quantum gravity, in its loop formulation or otherwise, has to date been of greater interest to philosophers because it appears to confront fundamental questions in a way that string theory, at least in its perturbative guise, does not — certainly, it does so more explicitly and in language more amenable to philosophers. Whereas perturbative string theory treats spacetime in an essentially classical way, canonical quantum gravity treats it as quantum-mechanical entity, at least to the extent of treating the geometric structure (as opposed to, say, the topological or differential structure) as quantum-mechanical. Furthermore, many of the issues facing canonical quantum gravity are also firmly rooted in conceptual difficulties facing the classical theory, which philosophers are already well acquainted with (e.g. via the  hole argument).