This chapter presents Bohr’s solution to the problem of permanence, based on a reinterpretation of his correspondence principle. The reinterpretation is grounded in Hankel’s principle of permanence, it elucidates the sense in which quantum mechanics (QM) can be considered a rational generalization of classical physics, and justifies the attribution to Bohr of a non-representationalist metasemantics for QM. After reviewing von Neumann’s use of Hankel’s principle in quantum logic (QL), the chapter revisits Weyl’s criticism that QL is not categorical because its connectives are not truth-functional.