 In classical thermodynamics, that a system possesses entropy is often attributed to the fact that in practice we are never able to give it a “complete” description (Jaynes 1967). When describing a cloud of gas, we do not specify values for the position and velocity of every molecule in it; we rather describe it using quantities, such as pressure and temperature, constructed as statistical measures over underlying, more finely grained quantities, such as the momentum and energy of the individual molecules. On one common construal, then, the entropy of the gas measures the incompleteness, as it were, of the gross description. (See the entry  Philosophy of Statistical Mechanics.)  In the attempt to take seriously the idea that a black hole has a true physical entropy, it is therefore natural to attempt to construct such a statistical origin for it. The tools of classical general relativity cannot provide such a construction, for it allows no way to describe a black hole as a system whose physical attributes arise as gross statistical measures over underlying, more finely grained quantities. Not even the tools of quantum field theory on curved spacetime can provide it, for they still treat the black hole as an entity defined entirely in terms of the classical geometry of the spacetime (Wald 1994). Any such statistical accounting, therefore, must come from a theory that attributes to the classical geometry itself a description based on an underlying, perhaps discrete collection of microstates, themselves describing the fine-grained dynamics of “entities”, presumably quantum in nature, underlying the classical spacetime description of the black hole. Note that any program aimed at “counting black-hole microstates” need not accept a subjectivist interpretation of entropy à la Jaynes. In any event, on any view of the nature of entropy, there arises a closely related problem, viz., to locate “where” black-hole entropy resides: inside, on, or outside the event horizon? See Jacobson et al. (2005) for a thoughtful dialogue among three eminent physicists with different point of views on the matter. 