 The collapse postulate occurs already in the general discussion at the fifth Solvay Conference in 1927 (see Bacciagaluppi and Valentini, 2009, 437–450). It is also found in Heisenberg’s The Physical Principles of the Quantum Theory, based on lectures presented in 1929 (Heisenberg, 1930a, 27; 1930b, 36). Von Neumann, in his reformulation of quantum theory a few years later, distinguished between two types of processes: Process 1:, which occurs upon performance of an experiment, and Process 2:, the unitary evolution that takes place as long as no measurement is made (von Neumann, 1932; 1955, §V.I). He does not take this distinction to be a difference between two physically distinct processes. Rather, the invocation of one process or the other depends on a somewhat arbitrary division of the world into an observing part and an observed part (see von Neumann,1932, 224; 1955, 420).