I'm currently reading Dirac's *Principles of Quantum Mechanics*, on page 36, he says: 

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> Another assumption we make connected to the physical interpretation of the theory is that, if a certain real dynamical variable $\xi$ is measured with the system in a particular state, the states into which the system may jump on account of the measurement are such that the original system is dependent on them.
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On what physical basis can we make this assumption and why is it reasonable?