Defeasible Logic¶
Defeasible Logic derives definite and provisional conclusions from typed rules, blocking defeaters and explicit priorities when evidence conflicts.
Core Idea¶
Defeasible Logic uses facts, strict and defeasible rules, blocking-only defeaters, an explicit superiority relation and proof tags. It can infer normal-case conclusions while distinguishing a reason to withhold q from a reason to prove ¬q. Strict inference from definite premises is separate from a rule used with only defeasibly supported premises.[^ref-0a8185911412]
Scope of Application¶
The original formal paper models an emu as definitely a bird, bird flight as a defeasible rule, heavy weight as a flight defeater and broken wings as an opposing rule that can be superior. An e-contract research example models premium discounts, special-order non-discounts and promotion non-discounts with explicit priority. The latter is an encoded illustrative policy, not a legal determination.[ref-0a8185911412][ref-44b01d99f663]
Clarity¶
heavy(X)↝¬flies(X) can stop a flight conclusion but does not establish nonflight. brokenWing(X)⇒¬flies(X) can support nonflight if it defeats the bird-flight rule. Definite +Δbird from a strict emu rule differs from provisional +∂flies; adding facts can retract only the latter sort of conclusion.[^ref-0a8185911412]
Manages Complexity¶
Rule types separate certainty, normality, doubt and contrary support. Priority makes conflicts inspectable, but the formal proof cannot validate whether a business or legal priority was properly chosen. Wrong facts or rule ordering produce precise but mistaken outputs.[ref-0a8185911412][ref-44b01d99f663]
Abstract Reasoning¶
Test definite derivability from facts and strict rules, then test applicable provisional support against all applicable contrary rules and defeaters. A superior rule can overcome an inferior contradictory rule; a defeater can only block. Report whether a literal is definitely or defeasibly provable or refuted, rather than returning an unqualified Boolean answer.[^ref-0a8185911412]
Knowledge Transfer¶
The same typed-rule and priority procedure operates in commonsense animal examples and e-contract policy illustrations. Their factual and institutional premises do not transfer. A basic Defeasible Logic proof is not itself a compliance certification or a synonym for every form of default logic. This named calculus is a kind of Formal System: its symbolic rules and tagged derivations are mechanical, while defeaters and priorities distinguish it from that broader class.
[^ref-0a8185911412]: G. Antoniou, D. Billington, G. Governatori and M. J. Maher, “Representation Results for Defeasible Logic,” original author manuscript, §§2.1–2.3, including bird/emu examples and proof tags. [^ref-44b01d99f663]: G. Governatori and D. H. Pham, “DR-CONTRACT: An Architecture for e-Contracts in Defeasible Logic,” original author-uploaded paper, illustrative discount rules and priorities; full platform access is limited.
Relationships to Other Abstractions¶
Current abstraction Defeasible Logic Domain-specific
Parents (1) — more general patterns this builds on
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Defeasible Logic is a kind of Formal System Prime
Defeasible Logic specializes a formal system with typed defeasible rules, priorities and tagged proofs.
Hierarchy paths (2) — routes to 2 parentless roots
- Defeasible Logic → Formal System → Formalization → Representation → Abstraction
- Defeasible Logic → Formal System → Formalization → Transformation → Function (Mapping)
Neighborhood in Abstraction Space¶
Defeasible Logic sits in a moderately populated region (52nd percentile for distinctiveness): it has near-neighbors but no dense thicket of look-alikes.
Family — Property Ontology & Code Smells (18 abstractions)
Nearest neighbors
- Denying the Antecedent — 0.87
- Legal Syllogism — 0.86
- Peirce's Law — 0.86
- Intuitionistic Type Theory — 0.85
- Principle of Explosion — 0.85
Computed from structural-signature embeddings · 2026-10-08