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The first of these principles was formulated by quantum physicists (such as Dirac) in the 1920's to fend off awkward questions such as How can a system suddenly jump from one state into another?'', or How does a system decide which state to jump into?''. As we shall see, the second principle is the basis for the mathematical formulation of quantum mechanics. The final principle is still rather vague. We need to extend it so that we can predict which possible states a system can jump into after a particular type of observation, as well as the probability of the system making a particular jump.