Cox's theorem states that any system of representing beliefs that satisfies certain criteria of reasonableness must be structurally equivalent to probability theory.
Along with the Dutch book argument, Cox's theorem is an argument for considering probability theory as a normative theory of reasoning.
- K. S. Van Horn (2003). "Constructing a Logic of Plausible Inference: a Guide To Cox's Theorem". International Journal of Approximate Reasoning 34 (1): 3-24. (PDF)