deviations from behavior the less basic theory leads one to expect.

But by 1975 a second motivation had become apparent—the hope that by revising or reformulating quantum mechanics as a theory of local beables one might remove ambiguity and arrive at increased precision. It is in this context that Bell now introduces a requirement of local causality. This differs in two ways from his earlier locality requirement. It is not a requirement on the world, but on theories of local beables: and it applies to theories that are probabilistic, with deterministic theories treated as a special case in which all probabilities are 0 or 1 (and densities are delta functions).[^1] Bell (\[1975\], \[1985\]) designs his requirement of local causality as a generalization of a requirement of local determinism met by Maxwell’s electromagnetic theory. In source-free Maxwellian electromagnetism, the local beables are the values of the electric and magnetic fields at each point $(\mathbf{x},t)$. This theory is locally deterministic because the field values in a space-time region are uniquely determined by their values at an earlier moment in a finite volume of space that fully closes the backward light cone of that region.

Local causality arises by generalizing to theories in 