 Here again, as with almost all the issues discussed up to this point in this entry regarding singularities and black holes, is an example of a sizable subculture in physics working on matters that have no clearly or even unambiguously defined physical parameters to inform the investigations and no empirical evidence to guide or even just constrain them, the parameters of the debate imposed by and large by the intuitions of a handful of leading researchers. From sociological, physical, and philosophical vantage points, one may well wonder, then, why so many physicists continue to work on it, and what sort of investigation they are engaged in. Perhaps nowhere else in general relativity, or even in physics, can one observe such a delicate interplay of, on the one hand, technical results, definitions and criteria, and, on the other hand, conceptual puzzles and even incoherence, largely driven by the inchoate intuitions of physicists. Not everyone views the situation with excitement or even equanimity, however: see Curiel (1999) for a somewhat skeptical discussion of the whole endeavor. 