  In 1964, John Bell proved that quantum mechanics is "unreasonable" (to use
Einstein's term): there are nonlocal bipartite quantum correlations. But they
are not the most nonlocal bipartite correlations consistent with relativistic
causality ("no superluminal signalling"): also maximally nonlocal
"superquantum" (or "PR-box") correlations are consistent with relativistic
causality. I show that---unlike quantum correlations---these correlations do
not have a classical limit consistent with relativistic causality. The
generalization of this result to all stronger-than-quantum nonlocal
correlations is a derivation of Tsirelson's bound---a theorem of quantum
mechanics---from the three axioms of relativistic causality, nonlocality, and
the existence of a classical limit. But is it reasonable to derive (a part of)
quantum mechanics from the unreasonable axiom of nonlocality?! I consider
replacing the nonlocality axiom with an equivalent axiom that even Bell and
Einstein might have considered reasonable: an axiom of local retrocausality.
