Epistemic Arithmetic

Instructor: Eric Pacuit (website)

ESSLLI 2025 • Bochum, Germany

July 28 - August 1, 2025

14:00 - 15:30 • HGB 50

This course examines epistemic extensions of formal arithmetic. We will begin by reviewing formal theories of arithmetic, including Peano Arithmetic, and Gödel's incompleteness theorems. With this foundation in place, we will study extensions of Peano Arithmetic that introduce an operator meant to represent the knowledge of an ideal mathematician. One of the key topics covered in the course is the Knower Paradox, which arises when the knowledge operator is treated as a predicate. We will analyze different versions of the paradox and evaluate proposed solutions. The course also explores attempts to formalize Gödel's disjunction, which asserts that either no algorithm can fully capture human mathematical reasoning or there are absolutely undecidable problems. While it will not be possible to cover all formal details of Gödel's theorems, our goal is to provide students with a clear understanding of the central ideas, the significance of the Knower Paradox, and the philosophical implications of the incompleteness theorems.

Day 1: Background: Formal Arithmetic and Godel's Incompleteness Theorems

July 28, 2025

Introduction to Godel's Theorems; Robinson’s Q; Peano Arithmetic PA; a brief introduction to Godel coding; Godel-Carnap fixed-point theorem; and a brief discussion of Godel’s incompleteness theorems

Day 2: Provability Logic, Lob's Theorem, Predicate Approaches to Modality

July 29, 2025

Diagonlization and Godel's Theorems; Derivability conditions; Lob's Theorem.

Day 3: The Knower Paradox

July 30, 2025

A brief introduction to the modal logic of provability; Predicate vs. operator approaches to modality; The Knower Paradox and variants

Day 4: More on the Knower Paradox; Doxastic and Epistemic Logic and Anti-Expert Paradoxes

July 31, 2025

More on the Knower Paradox; A primer on epistemic and doxastic logic, anti-expert paradoxes; more on predicate approaches to modality

Day 5: Godel's Disjunction

August 1, 2025

More on predicate approaches to modality; Epistemic arithmetic; Towards a proof of Godel’s Disjunction in epistemic arithmetic.