 It should not be supposed that Einstein’s notion of a complete theory included the requirement that the theory should be deterministic. Rather, he required certain conditions of separability and locality for composite systems consisting of separated component systems: each component system separately should be characterized by its own properties (its own ‘being-thus,’ as Einstein put it — ‘So-sein’ in German), and it should be impossible to alter the properties of a distant system instantaneously (or the probabilities of these properties) by acting on a local system. In later analyses, notably in Bell’s argument for the nonlocality of quantum correlations, it became apparent that these conditions, suitably formulated as probability constraints, are equivalent to the requirement that statistical correlations between separated systems should be reducible to probability distributions over common causes (deterministic or stochastic) in the sense of Reichenbach. (See the entries on  Bell’s theorem  and  Reichenbach’s common cause principle.)