 As in the case of singularities, these alternative definitions of black holes raise philosophical questions about the relations among the different definitions that attempt to capture different aspects of, intuitively speaking, the “same kind” of physical object. One can, for instance, view the standard definition of a black hole, with its global event horizon, as an extreme idealization of an isolated system (one with no neighboring systems at all), and the definitions based on isolated or trapping horizons as trying to capture a more general, less idealized representation of an isolated system, one that has neighboring systems at a finite remove, or a representation of a system that may be non-trivially interacting with other systems. For the looser, less precise definitions used by astrophysicists, for example, and some of the gestures at definitions proposed in some programs of quantum gravity, however, it is difficult to know how even to begin to compare them to the precise global and quasi-local ones. It is simply not clear that the same type of physical system is being characterized. 