" Rule 1, by dictating how a quantum system changes in time, plays the same essential role in the theory that Newton’s laws of motion played in pre-quantum physics. Like Newton’s laws, Rule 1 is deterministic. It takes an input state and evolves it to a definite output state at a later time. This means it takes input states which are constructed as superpositions to output states which are similarly constructed from superpositions. Probability plays no role. But measurements, as described by Rule 2, do not evolve superpositions to other superpositions. When you measure some quantity, like pet preference or position, you get a definite value. And afterward the state is the one corresponding to that definite value. So even if the input state is a superposition of states with definite values of some observable quantity, the output state is not, as it corresponds to just one value. Rule 2 does not tell you what the definite value is; it only predicts probabilities for the different possible outcomes to occur. But these probabilities are not spurious; they are part of what quantum mechanics predicts. Rule 2 is essential, because that is how probabilities enter quantum mechanics. And probabilities are essential in many cases; they are what experimentalists measure. However, quantum mechanics requires that Rule 1 and Rule 2 never be applied to the same process, because the two rules contradict each other. This means we must always distinguish measurements from other processes in nature. "
― Lee Smolin , Einstein's Unfinished Revolution: The Search for What Lies Beyond the Quantum