- A query
A human asking Wigner and his friend about the states they assign has certain and complete information about both Wigner's pure state of the lab and his friend's pure state of the device. How do these certainties show up (as certainties) in the pure state of ''lab + Wigner + his friend''? Since the pure state is supposed to be a state of maximal knowledge these certainties should be deducible deterministically from this state.Wigner has a mixed state for his friend's experimental device because he has maximal possible knowledge (a pure state) for the Lab as a whole. The friend does not track the lab as a whole and thus he can have maximal knowledge (a pure state) for the device.