What Is Ontic and What Is Epistemic in the Quantum Mechanics of Spin?

Entropic Dynamics (ED) provides a framework that allows the reconstruction of the formalism of quantum mechanics by insisting on ontological and epistemic clarity and adopting entropic methods and information geometry. Our present goal is to extend the ED framework to account for spin. The result is a ψ in which the ontology consists of a particle described by a definite position plus a discrete variable that describes Pauli's peculiar two-valuedness. The resulting dynamics of probabilities is, as might be expected, described by the Pauli equation. What may be unexpected is that the generators of transformations-Hamiltonians and angular momenta, including spin, are all granted clear epistemic status. To the old question, 'what is spinning?' ED provides a crisp answer: .