The Path Integral Formulation of Gravitational Thermodynamics
  The first objective of this article is to show that the black hole partition function can be placed on a firm logical foundation by enclosing the black hole in a spatially finite "box" or boundary. The presence of the box has the effect of stabilizing the black hole and yields a system with a positive heat capacity. The second objective of this article is to explore the origin of black hole entropy. This is accomplished through the construction of a path integral expression for the density matrix for the gravitational field, and through an analysis of the connection between the density matrix and the black hole density of states. Our results suggest that black hole entropy can be associated with an absence of certain "inner boundary information" for the system. (Based on the talk presented by J.D. Brown at the conference "The Black Hole 25 Years After", Santiago, Chile, January 1994.) 
J.D. Brown and J.W. York