 Zurek 2005 offers a new twist to the Born rule derivation based on the permutation symmetry of states corresponding to worlds with equal measures of existence. He considered entangled systems and relies on “envariance” symmetry: a unitary evolution of a system which can be undone by the unitary evolution of the system it is entangled with. Zurek assumes that a manipulation of the second system does not change the probability of the measurement on the first system. The swap of the states of the system swaps the probabilities of the outcomes, because the outcomes are correlated with the other systems, where nothing has been changed. Since the swaps of the two systems lead to the original state, the probabilities should be unchanged, but they have swapped, so they must be equal.