  Superquantum ("PR-box") correlations, though designed to respect relativistic
causality, violate relativistic causality in the classical limit. Generalizing
to all stronger-than-quantum bipartite correlations, I derive Tsirelson's bound
from the axioms of nonlocality, relativistic causality and the existence of a
classical limit. This derivation of Tsirelson's bound does not assume quantum
mechanics yet suggests how Hilbert space is implicit in quantum correlations.
