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The passage around this formula

Start with the settled part, because it is smaller than most accounts suggest and it is worth stating precisely. Quantum mechanics gives the Born rule: the probability of a measurement outcome is the squared modulus of a probability amplitude, $P(a) = |\langle a \mid \psi \rangle|^2$. for a system in state |$\psi$$\rangle$ measured in a basis containing |a$\rangle$ . Every interpretation discussed here reproduces this rule for every experiment that has actually been performed. That includes Bell tests. John Bell showed in 1964 that any theory obeying two assumptions — that a measurement’s outcome depends only on a hidden variable carried by the particle and on the local setting of its own detector, not on the distant…

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