  Even after over 150 years of discussion, the interpretation of the second law
of thermodynamics continues to be a source of confusion and controversy in
physics. This confusion has been accentuated by recent challenges to the second
law and by the difficulty in many cases of clarifying which formulation is
threatened and how serious the implications of a successful challenge would be.
To help bring clarity and consistency to the analysis of these challenges, the
aim of this paper is to suggest a simple formulation of the deep physics of the
second law, and to point out how such a statement might help us organize the
challenges by level of seriousness. We pursue the notion that the second law is
ultimately a restriction operating directly on the dynamics of information, so
the existence of this law can be traced to the need for a system of
"information bookkeeping" that is independent of the bookkeeping for energy.
Energy and information are related but independent, so the dynamical
restrictions for one cannot be derived from those for the other. From this
perspective, we also suggest the possibility that the foundation of the second
law may be linked to the finite capacity of nature to store information about
its own state.
