  Most physicists agree that the Einstein-Podolsky-Rosen-Bell paradox
exemplifies much of the strange behavior of quantum mechanics, but argument
persists about what assumptions underlie the paradox. To clarify what the
debate is about, we employ a simple and well-known thought experiment involving
two correlated photons to help us focus on the logical assumptions needed to
construct the EPR and Bell arguments. The view presented in this paper is that
the minimal assumptions behind Bell's inequality are locality and
counterfactual definiteness, but not scientific realism, determinism, or hidden
variables, as is often suggested. We further examine the resulting constraints
on physical theory with an illustration from the many-worlds interpretation of
quantum mechanics -- an interpretation that we argue is deterministic, local,
and realist, but that nonetheless violates the Bell inequality.
