 Another line of investigation leading to Bell’s Theorem was the investigation of quantum mechanical entangled states, that is, quantum states of a composite system that cannot be expressed either as products of quantum states of the individual components, or as mixtures of product states. That quantum mechanics admits of such entangled states was discovered by Erwin Schrödinger (1926) in one of his pioneering papers, but the significance of this discovery was not emphasized until the paper of Einstein, Podolsky, and Rosen (1935). They examined correlations between the positions and the linear momenta of two well separated spinless particles and concluded that in order to avoid an appeal to nonlocality these correlations could only be explained by “elements of physical reality” in each particle — specifically, both definite position and definite momentum — and since this description is richer than permitted by the uncertainty principle of quantum mechanics their conclusion is effectively an argument for a hidden variables interpretation.  [2]  See also the entry on the  Einstein-Podolsky-Rosen paradox.