A mixed state is when the system is actually in one state or another, but you just don't know which, and you use probabilities to describe your uncertainty. I'm referring to a mixed state of the entire system. I want an actual example. Can you think of some? Note I wasn't describing mixed state of a subsystem only, obtained by tracing over the rest of the system that aren't measured, with the system as a whole being in a pure state (like in case of decoherence) My concern is, how do you know the system is actually in one state even prior to measurement or observation? (ever heard of the Bertelsmann's socks argument in Bell's Theorem?) Mixed state argument is like that of Bertelsmann's socks or properties/state existing even before observation. What's the proof or experimental example of this?