The Two Bell's Theorems of John Bell

Understand

Many of the heated arguments about the meaning of "Bell's theorem" arise because this phrase can refer to two different theorems that John Bell proved, the first in 1964 and the second in 1976.

• His 1964 theorem is the incompatibility of quantum phenomena with the dual assumptions of locality and determinism.
• His 1976 theorem is the incompatibility of quantum phenomena with the unitary property of local causality.
• This is contrary to Bell's own later assertions, that his 1964 theorem began with that single, and indivisible, assumption of local causality (even if not by that name).