  The third law of thermodynamics is formulated precisely: all points of the
state space of zero temperature $\Gamma_0$ are physically adiabatically
inaccessible from the state space of a simple system. In addition to implying
the unattainability of absolute zero in finite time (or "by a finite number of
operations"), it admits as corollary, under a continuity assumption, that all
points of $\Gamma_0$ are adiabatically equivalent. We argue that the third law
is universally valid for all macroscopic systems which obey the laws of quantum
mechanics and/or quantum field theory. We also briefly discuss why a precise
formulation of the third law for black holes remains an open problem.
