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What is Logic?

What is Logic?

This is the homepage of my draft textbook, What is Logic?. The DRAFT status of this book is very important: Portions of it are substantially incomplete. If, however, you find any errors, please let me know.

There is a supplementary series of youtube videos covering content in this book.

Recent updates:

  • 2026-03-27: Uploaded draft 2026-03-27, which moves some material from Section 4.2 into its own, new section (Section 4.4), enlarges Section 4.3, and adds more terms to the index.
  • 2026-03-17: Uploaded draft 2026-03-17, which cleans up a bunch of small typos in many sections, substantially enlarges Section 8.6.6 on the soundness of natural deduction with respect to the natural predicate logic models, and rearranges Chapter 4 in order to add material on logic in ancient China (Section 4.3).
  • 2026-02-12: Uploaded draft 2026-02-12, which distinguishes classical from non-classical valuations at the start of Section 7.4, corrects the annotations for example 7.5.15, and added some info to Chapter 21.