 What is now known as the two-state vector formalism was first proposed by Watanabe (1955), and then rediscovered by Aharonov, Bergmann, and Lebowitz (1964). The proposal is that the usual forward evolving quantum state contains insufficient information to completely specify a quantum system; rather the forward evolving state must be supplemented by a backward evolving state to provide a complete specification. Thus, according to the two-state vector formalism, the complete quantum description contains a state vector that evolves forward in time from the initial boundary condition towards the future, and a state vector that evolves backward in time from the future boundary condition towards the past. It is only a combination of complete measurements at both initial and final boundaries that can provide a complete specification of a quantum system. The two-state vector formalism is empirically equivalent to standard quantum mechanics (Aharonov &amp; Vaidman 1991, 2008).