I am trying to learn quantum mechanics and I have a question. Landau, in his quantum mechanics book says that it is in principle impossible to formulate basic concepts of quantum mechanics without using classical mechanics. But I learned a bit about consistent histories and I think it makes sense to talk about closed quantum mechanical systems using this language if I didn't understand something wrong.
Then I have listened to a lecture video of a very famous physicist (Milner prize winner) who says that quantum mechanics does not make sense if you can't go out of the system and measure it using an infinite apparatus (he considers decoherence and when you calculate expectation value of an observable you see that you can talk about classical probabilities, that is non-diagonal terms disappear). Hence wavefunction of universe does not make sense. I believe what he says is actually same as Landau.
Is it meaningful to talk about closed systems which does not interact with outside such as universe? What does framework of consistent histories says about this?