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Bohrification: From classical concepts to commutative algebras

The Bohrification program is an attempt to interpret Bohr's mature doctrine of classical concepts as well as his earlier correspondence principle in the operator-algebraic formulation of quantum theory pioneered by von Neumann. In particular, this involves the study of commutative C*-algebras in relationship to noncommutative ones. This relationship may take the form of either exact Bohrification, in which one studies commutative unital C*-subalgebras of a given noncommutative C*-algebra, or asymptotic Bohrification, which involves deformations of commutative C*-algebras into noncommutative ones. Implementing the doctrine of classical concepts, exact Bohrification is an appropriate framework for the Kochen-Specker Theorem and for (intuitionistic) quantum logic, culminating in the topos-theoretic approach to quantum mechanics that has been developed since 1998. Asymptotic Bohrification was inspired by the correspondence principle and forms the right conceptual and mathematical framework for the explanation of the emergence of the classical world from quantum theory (incorporating the measurement problem and the closely related issue of spontaneous symmetry breaking). The Born rule may be derived from both exact and asymptotic Bohrification, which reflects the Janus faces of probability as applying to individual random events and to relative frequencies, respectively. We review the history, the goals, and the achievements of this program so far.