  Relativistic causality (RC) is the principle that no cause can act outside
its future lightcone, but any attempt to formulate this principle more
precisely will depend on the foundational framework that one adopts for quantum
theory. Adopting a histories-based (or "path integral") framework, we relate RC
to a condition we term "Persistence of Zero" (PoZ), according to which an event
$E$ of measure zero remains forbidden if one forms its conjunction with any
other event associated to a spacetime region that is later than or spacelike to
that of $E$. We also relate PoZ to the Bell inequalities by showing that, in
combination with a second, more technical condition it leads to the quantal
counterpart of Fine's patching theorem in much the same way as Bell's condition
of Local Causality leads to Fine's original theorem. We then argue that RC per
se has very little to say on the matter of which correlations can occur in
nature and which cannot. From the point of view we arrive at, histories-based
quantum theories are nonlocal in spacetime, and fully in compliance with
relativistic causality.
