 Indeed, relativistically mass just is energy, so at least the First Law seems already to be more than just formal analogy. Also, the fact that the state of a stationary black hole is entirely characterized by only a few parameters, completely independent of the nature and configuration of any micro-structures that may underlie it (e.g., those of whatever collapsed to form the thing), already makes it sound more than just a little thermodynamical in character. (Recall the discussion of the No Hair Theorems in  section 3.2  above.) Still, although the analogy is extremely suggestive in toto, to take it seriously would require one to assign a non-zero temperature to a black hole, which, at the time Bardeen, Carter and Hawking first formulated and proved the laws in 1973, almost everyone agreed was absurd. All hot bodies emit thermal radiation (like the heat given off from a stove, or the visible light emitted by a burning piece of charcoal); according to general relativity, however, a black hole ought to be a perfect sink for energy, mass, and radiation, insofar as it absorbs everything (including light), and emits nothing (including light). So it seems the only temperature one might be able to assign it would be absolute zero. (See  section 5.4.2  below for more detailed arguments to this effect.) 