is supposed to, one needs to assume the very thing that is to be achieved.

Quantum decoherence arises from i) the fact that the inevitable interactions between a system and its environment typically lead to entanglement between the two and ii) the fact that the states of the environment entangled with the different states of the system, usually become, very quickly and to a very good approximation, orthogonal among themselves. These facts are often described in a more “poetic” language which, sometimes, is not straightforward to interpret formally. For example, Crull defines decoherence as follows:

> \[I\]t is a dynamical process whereby a system’s phase relations in particular bases become decohered or randomized by commuting \[sic\] with external (environmental) degrees of freedom.[^1] [@Crull p. 1021]

However, it is rather unclear what lies behind the idea that the system’s phase relations become “decohered or randomized.” Although one might have some intuitive feeling about things becoming random when they are too complicated, the fact is that the unitary evolution provided by the Schrödinger equation contains absolutely nothing random. Moreover, no clear, unambiguous and universally accepted definition of coherence (which is supposed to get lost in the process) is available, For example, in 