#### Fredrik

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That follows immediately from the Born rule, so I guess you're asking why the Born rule holds. The Born rule is part of the definition of QM, so what you're really asking is why QM is a good theory (or rather, a good framework in which we can define theories of matter and interactions). The only thing that can answer that is a better theory.Fredrik, as stevendaryl says, my question is not about the math (I understand it). My question is if there is any reason that can be stated in a few words (I dont want to use the phrase "physical reason" but perhaps it helps you to find what Im asking) in order to explain why the probability to go from [itex]\psi[/itex] to [itex]\varphi[/itex] and the probability to go the other way round are the same.