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

## TL;DR

This paper explores the nature of spin in quantum mechanics using a framework that distinguishes between what is real and what is just our knowledge.

## Contribution

The paper extends the Entropic Dynamics framework to include spin, offering a realist ψ-epistemic model with clear ontological and epistemic distinctions.

## Key findings

- The model uses a particle's position and a discrete variable to describe spin.
- The dynamics of probabilities are governed by the Pauli equation.
- Spin and related quantities are given clear epistemic status, not ontic.

## Abstract

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 realist ψ-epistemic model 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: nothing is spinning.