🔭 Methodeutics

The logic of inquiry. Statistics formalizes induction: how to generalize from observations. Methodeutics formalizes abduction: how to generate the next question. Peirce named both the mode (1878) and the field (1902). This is the textbook.

Modes of reasoning

Everyone does abduction. Doctors call it diagnosis. Engineers call it debugging. Detectives call it deduction (wrong, but Doyle didn't read Peirce). The pieces have always existed: philosophy, cognitive science, program analysis, causal inference, sequential testing.

How to read this book

Prerequisites: comfort reading short Python programs, and one encounter with probability at the level of a fair coin. Everything else the book uses (separation logic, martingales, type constructors) is taught where it is first needed.

The chapters are a single argument and read best in order, but there are three doors:

• From software or engineering: start at Chapter 4. The mechanic will feel like debugging because it is debugging. Circle back to Part I when you want to know what the operation is called and why it took a century to name.
• From statistics or data science: start at Chapter 9, where your field's snapshot habit fails three times in public. Then read Part II to see where the hypotheses come from.
• From philosophy: start at Chapter 1 and the appendices, which include the primary Peirce texts in full.

Each chapter ends with five exercises: a warm-up, two worked problems, a computer exploration (💻), and an open-ended field problem (★) that points the chapter's tool at a system you already own.

Chapters

Each chapter opens with a system that breaks under the previous chapter's tools. The breakage motivates the new concept. The shape of the failure names the next chapter.

Part I: The trichotomy

Ground-clearing. What abduction is, why it trades certainty for fertility, and the evidence that six fields keep rediscovering it under different names. No mathematics yet; the object of study has to exist before it can be measured.

Part II: The primitives

The mechanical core. A diff between expected and observed generates hypotheses, bi- and tri-abduction make the generation algorithmic, framing decides what the diff runs over, and economy decides which candidate to test first. By the end of Part II you can generate and rank. You cannot yet score a test.

Part III: The evidence

What comes back from the experiment, read while it is still arriving. Evidence becomes a trajectory, a running bet against the hypothesis. Trajectories sort into four bins, the bins drive a hypothesis graph, and composed trajectories either converge or kill.

Part IV: The machine

Assembly. The generate-test-update loop hardened against self-deception, validators that accept proofs and counterexamples alongside data, the full pipeline, and one real system taken apart in public.

Appendix: Primary Sources

between vision and root
blooms branches and shoots

prune the duds
thence more buds

clues for felling
perfuse its telling

do trees need an arbor
in light, soil and water?

Peirce's vocabulary

Terms coined or repurposed by Charles Sanders Peirce (1839–1914) that do work in this textbook.

Status & citation

Draft. All 17 chapters written, with exercises. Last revised July 2026. Corrections welcome: kimjune01@gmail.com.

Companion package: pip install methodeutics — the Chapter 10 e-value bankroll as a library (source).

@misc{kim2026methodeutics,
  author = {Kim, June},
  title  = {Methodeutics: The Logic of Inquiry},
  year   = {2026},
  url    = {https://june.kim/reading/methodeutics/},
  note   = {Online textbook, draft}
}