Measurement

The postulate 2 told us how the system evolves in a closed system, but that is totally hidden from us and we cant really make use of it until we know about The evolution of systems which don’t interact with the rest of the world is all very well, but there must also be times when the experimentalist and their experimental equipment – an external physical system in other words – observes the system to find out what is going on inside the system, an interaction which makes the system no longer closed, and thus not necessarily subject to unitary evolution.

SInce the 3rd postulate is about measuring it is obvious thay upon measuring we must be in a position to differentiate between the different outcomes otherwise the measurment will not make much sense.