---
title: "MYCIN"
type: "entity"
entity_kind: "concept"
status: "hub"
canonical_name: "MYCIN"
aliases: ["EMYCIN"]
first_seen: "2026-10-17T00:00:00.000Z"
writer_model: "claude-opus-4-8"
connects_to: ["backward chaining","expert systems","certainty factors","Edward Shortliffe","Bruce Buchanan"]
seek_code_commit: "d40ee39"
---


MYCIN was a rule-based expert system built at Stanford in the early-to-mid 1970s
to diagnose bacterial infections and recommend antibiotic therapy, using roughly
600 IF-THEN rules and a "certainty factor" calculus for reasoning under
uncertainty. Its domain-independent shell, EMYCIN, was extracted by William van
Melle and became a template for later expert systems. It is one of the canonical
early expert systems and the paradigm case of backward-chaining inference.

Vault relevance: MYCIN is the load-bearing worked example across two of the
vault's threads — the inference-engine lineage (it is the concrete
backward-chaining system behind [[claim-inference-classic-ai-engines]]) and the
rule-based-AI-to-Bayesian-networks arc (its certainty factors are the ad hoc
non-Bayesian scheme whose incoherence Heckerman later proved). Its control
structure also anchors the finding that ACH's matrix elimination is a
*mechanism-level false friend* for backward chaining, not a twin.

## References
- [[claim-mycin-backward-chaining-is-goal-directed-recursive-subgoal-search]] — its control structure, from van Melle's own 1984 chapter (Tier 1)
- [[claim-inference-classic-ai-engines]] — MYCIN as the paradigm backward-chaining engine; EMYCIN as the reusable shell
- [[claim-mycin-certainty-factors-non-bayesian-uncertainty]] — its certainty-factor uncertainty calculus, CF = MB − MD
- [[claim-heckerman-1986-certainty-factors-require-independence-assumptions]] — the 1986 critique of MYCIN's CF scheme
- [[observation-ach-backward-chaining-mechanism-comparison-is-false-friend]] — MYCIN's backward chaining read against Heuer's ACH
- Creators: [[entity-edward-shortliffe]] and [[entity-bruce-buchanan]]; EMYCIN and the cited control-structure chapter are William van Melle's work