Epistemic Arithmetic
Instructor: Eric Pacuit (website)
ESSLLI 2025 • Bochum, Germany
July 28 - August 1, 2025
14:00 - 15:30 • TBA
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
Topics to be covered: Robinson’s Q; Peano Arithmetic PA; Heyting Arithmetic HA; a brief introduction to Godel coding; Godel-Carnap fixed-point theorem; and a brief discussion of Godel’s incompleteness theorems.
Day 2: Provability Logic and Epistemic Arithmetic
July 29, 2025
Topics to be covered: Derivability conditions; A brief introduction to the modal logic of provability; Introduction to epistemic logic; Epistemic arithmetic; Predicate vs. operator approaches to modality.
Day 3: The Knower Paradox
July 30, 2025
Topics to be discussed: Variants and responses to the Knower Paradox
Day 4: Godel's Disjunction I
July 31, 2025
Topics to be discussed: Towards a proof of Godel’s Disjunction in epistemic arithmetic; formal theories of truth; absolute provability; philosophical implications.
Day 5: Godel's Disjunction II
August 1, 2025
Topics to be discussed: Towards a proof of Godel’s Disjunction in epistemic arithmetic; formal theories of truth; absolute provability; philosophical implications.