 One line of investigation in the prehistory of Bell’s Theorem is Bell’s examination of the hidden-variables program. This program involves supplementation of the quantum mechanical state of a system by further “elements of reality”, or “hidden variables”, the incompleteness of the quantum state being the explanation for the statistical character of quantum mechanical predictions concerning the system. A pioneering version of a hidden variables theory was proposed by Louis de Broglie in 1926–7 (de Broglie 1927, 1928), and revived by David Bohm in 1952 (Bohm 1952; see also the entry on  Bohmian mechanics).