 The conceptual difficulties basically follow from the nature of the gravitational interaction, in particular the equivalence of gravitational and inertial mass, which allows one to represent gravity as a property of spacetime itself, rather than as a field propagating in a (passive) spacetime background. When one attempts to quantize gravity, one is subjecting some of the properties of spacetime to quantum fluctuations. For example, in canonical quantizations of gravity one isolates and then quantizes geometrical quantities (roughly the intrinsic and extrinsic curvature of three dimensional space) functioning as the position and momentum variables. Given the uncertainty principle and the probabilistic nature of quantum theory, one has a picture involving fluctuations of the geometry of space, much as the electric and magnetic fields fluctuate in QED. But ordinary quantum theory presupposes a well-defined classical background against which to define these fluctuations (Weinstein, 2001a, b), and so one runs into trouble not only in giving a mathematical characterization of the quantization procedure (how to take into account these fluctuations in the effective spacetime structure?) but also in giving a conceptual and physical account of the theory that results, should one succeed. For example, a fluctuating metric would seem to imply a fluctuating causal structure and spatiotemporal ordering of events, in which case, how is one to define equal-time commutation relations in the quantum theory? (See the section on the Lagrangian formulation in the entry on  quantum field theory.)  