Heisenberg cut

The original "Copenhagen" way of describing the collapse process was tied to a mysterious thing called the "Heisenberg cut". Everything lying "below" this cut was supposed to be described in the mathematical language of quantum mechanics, whereas everything lying "above" the cut was described either in the language of classical physics, or in psychological or mental terms. […] A principal move made by von Neumann was to show that the Heisenberg cut could be moved all the way up, so that reality was unambiguously separated into a purely mental part, and a part described in the mathematical language of quantum mechanics.