  We give a review, in the style of an essay, of the author's 1998
matter-gravity entanglement hypothesis which, unlike the standard approach to
entropy based on coarse-graining, offers a definition for the entropy of a
closed system as a real and objective quantity. We explain how this approach
offers an explanation for the Second Law of Thermodynamics in general and a
non-paradoxical understanding of information loss during black hole formation
and evaporation in particular. It also involves a radically different from
usual description of black hole equilibrium states in which the total state of
a black hole in a box together with its atmosphere is a pure state -- entangled
in just such a way that the reduced state of the black hole and of its
atmosphere are each separately approximately thermal. We also briefly recall
some recent work of the author which involves a reworking of the string-theory
understanding of black hole entropy consistent with this alternative
description of black hole equilibrium states and point out that this is free
from some unsatisfactory features of the usual string theory understanding. We
also recall the author's recent arguments based on this alternative description
which suggest that the AdS/CFT correspondence is a bijection between the
boundary CFT and just the matter degrees of freedom of the bulk theory.
