 For Heisenberg, the application of quantum mechanics requires a ‘cut’ between the system to be described quantum mechanically, and what is to be considered external to the system and is to be treated classically. Indeed, if one were to apply quantum mechanics to the entire universe, one would have a perfectly closed system in which nothing would ever happen. But Heisenberg places special emphasis on the idea that any special system must be describable using quantum mechanics (indeed, that such a system is in principle always able to display interference effects if placed under the appropriate  conditions[33]).  Self-consistency of the theory then requires the arbitrary movability of the cut away from the system. (The most detailed presentation of these ideas is in Heisenberg’s draft reply to the  Einstein–Podolsky–Rosen argument  – see Crull and Bacciagaluppi (2011) in the  Other Internet Resources.)