---
slug: defeasibility
name: Defeasibility
one_liner: "The modal property of a rule, inference, or claim that holds only BY DEFAULT: its conclusion follows unless-and-until a sufficient countervailing reason (a defeater) overrides it, so that new information can withdraw a previously-drawn conclusion without the rule being thereby falsified."
type: prime
schema_version: 2
status: density_v2_draft
categories: [structural, logical, legal]
origin_domain: logic
alternate_origin_domains: [law, artificial_intelligence, ethics, linguistics]
aliases: [defeasible_reasoning, rebuttable_presumption, prima_facie, default_reasoning, non_monotonic_reasoning, holds_unless_overridden]
related: []
solution_archetypes: []
review_flags:
- "OPERATOR-DERIVED (op-06 modalization, flavor 6c mandatory→default) — the modal structure obtained by taking a hard/absolute rule and asking what holds when the rule is loosened from must-always to holds-by-default-unless-overridden. Convergent discovery ×6 across `mutual_exclusion`, `correspondence_principle`, `coverage_reachability`, `constraint`, `uniformitarianism`, `epistemic_justice`. Discovered 2026-07-03. STRONG-MODERATE hold (verified absent from corpus; established named construct across logic/AI — Reiter's default logic, Pollock's defeaters — law — rebuttable presumption — and ethics — Ross's prima facie duties). STATUS: HOLD pending owner keep/cut."
- "Created 2026-07-03; STUB ONLY (frontmatter + Core Idea + Notes). SI/SF/similarity are PROPOSER ESTIMATES pending triangulation. Body sections (Structural Signature onward) NOT written."
- "GENUS-vs-SPECIAL-CASE screen required: `mandatory_vs_default_norms` is PRESENT and covers the deontic-NORMS case (which norms are mandatory vs default/overridable). Defeasibility is proposed as the GENUS — the holds-unless-overridden structure of any rule, inference, or factual claim (including inferences and presumptions, not just norms). Confirm it is the genuine general form and not a restatement of the norms special case; screen also vs any presumption / non-monotonic-reasoning prime and vs `counterfactual_reasoning`."
- "Phase-C action: re-similarity-check vs `mandatory_vs_default_norms` (the special case), `constraint`, `epistemic_justice`, and any presumption / non-monotonic-inference prime; if kept, draw a subsumption edge defeasibility → mandatory_vs_default_norms (norms as the deontic instance); resolve whether defeasible REASONING (the inference pattern) and defeasible RULES/RIGHTS (the object) are one head (recommend one); write a v1 draft and sync the one_liner across v1/v2 per CLAUDE.md before merge."
substrate_independence:
  composite_substrate_independence: 4
  domain_breadth: 5
  structural_abstraction: 4
  transfer_evidence: 5
  reasoning: 'The bare structure — a conclusion that follows by default and is withdrawable by a sufficient countervailing reason, with the withdrawal leaving the rule itself intact — is a genuinely substrate-neutral modal form. It recurs wherever a rule or inference is loosened from must-always to holds-unless-defeated: default and non-monotonic logic in AI (Reiter, Pollock), rebuttable presumptions and defeasible rights in law, prima facie duties in ethics (Ross), defeasible implicature in linguistics, and everyday defaults-with-exceptions. These are structurally-unlike siblings sharing only the abstract anatomy (a default conclusion + rebutting/undercutting defeaters + non-monotonicity + a burden-shifting default-until-challenged structure), so domain_breadth scores 5 and transfer evidence 5 (independently named canonical constructs in logic, AI, law, and ethics). Structural abstraction is 4 because the core is a pure statement about the modal status of a conclusion (holds-unless-overridden, distinct from holds-necessarily and holds-only-if-established) requiring no domain substance; it stays just short of 5 because "sufficient countervailing reason" carries a mild evaluative residue about what counts as a defeater.'
similarity_to_nearest_existing_prime:
  slug: mandatory_vs_default_norms
  similarity: 0.62
structural_framed:
  label: structural
  aggregate: 0.35
  criteria: {vocab_travels: 0.7, evaluative_weight: 0.35, institutional_origin: 0.35, human_practice_bound: 0.25, import_vs_recognize: 0.4}
  rationale: 'Reads structural. The core claim — a conclusion that holds by default and can be overridden by a sufficient defeater without the rule being falsified — is recognized rather than imported, and travels cleanly from legal presumptions into logic/AI non-monotonic inference, ethical prima facie duties, and everyday defaults without carrying courtroom or deontic vocabulary. The frame residue is light: "defeater," "presumption," and "sufficient reason" import a mild evaluative and quasi-legal cast (keeping evaluative_weight, institutional_origin, and import_vs_recognize off the floor), but the underlying modal structure — default-plus-defeaters plus non-monotonicity plus burden-shifting — is substrate-neutral. Comparable framing to its proposed special case `mandatory_vs_default_norms`, slightly less institution-bound because it also covers inferences and factual presumptions, not only norms.'
phase_b_passes_completed: []
---
# Defeasibility

## Core Idea

Defeasibility is the modal property of a rule, inference, or claim that holds *by default* rather than *absolutely*: its conclusion follows unless-and-until a sufficient countervailing reason — a *defeater* — overrides it, and when a defeater applies the conclusion is withdrawn *without the rule being thereby falsified*. It is reached from a hard, absolute, monotonic rule by loosening the modal force from "must always hold, no matter what further is learned" to "holds by default, provided nothing overrides it." The feature present at the defeasible level but absent at the absolute rule is precisely an *override channel*: an absolute rule has no vocabulary for "this normally follows but here it doesn't," because within a monotonic regime no new information can retract a licensed conclusion; a defeasible rule makes that retraction a first-class, structured possibility.

The load-bearing anatomy is fourfold. First, a *default conclusion* that follows unless a defeater applies — the presumption sits in force until challenged. Second, *defeaters* of two kinds: *rebutting* defeaters, which argue directly for the opposite conclusion, and *undercutting* defeaters, which do not argue the opposite but remove the *support* connecting premise to conclusion (the ground for believing it no longer holds, even though nothing positively contradicts it). Third, *non-monotonicity*: adding information can withdraw a conclusion that was correctly drawn before — the set of licensed conclusions does not grow monotonically with the premises, as it must under classical entailment. Fourth, a *burden-shifting* structure: the default holds until someone or something discharges the burden of producing a defeater, so the modal status is asymmetric — silence favors the default. What distinguishes defeasibility from an absolute rule is that no accumulation of new information can override the absolute rule (it is monotonic and exceptionless); a defeasible rule is *built to be overridden* under the right conditions, and being overridden is a normal operating mode, not a refutation.

Read structurally, the same holds-unless-overridden shape appears wherever a rule is loosened from mandatory to default, and the siblings are genuinely unlike. In *logic and AI*, it is default logic and non-monotonic reasoning (Reiter's defaults, Pollock's rebutting/undercutting defeaters) — inferences licensed unless a more specific rule or contrary evidence blocks them. In *law*, it is the rebuttable presumption and the defeasible right — a fact taken as established, or an entitlement held, until sufficient contrary showing shifts the burden. In *ethics*, it is Ross's *prima facie* duties — obligations that genuinely bind but can be overridden by a weightier competing duty in the concrete case, without ceasing to be duties. In *linguistics*, it is defeasible implicature — a default reading cancellable by context. Across these, the shared abstract engine is a default conclusion plus a structured override channel plus non-monotonicity; the legal presumption and the logical default are species of one genus.

## Notes

**Provenance.** Produced by operator 06 (modalization), flavor 6c (mandatory→default), as the modal structure that appears when a hard/absolute rule is loosened from must-always to holds-by-default-unless-overridden. Surfaced as a *convergent* discovery (×6) across six unlike inputs — `mutual_exclusion`, `correspondence_principle`, `coverage_reachability`, `constraint`, `uniformitarianism`, and `epistemic_justice` — each of which, when its rule was softened from absolute to default, pointed at the same holds-unless-defeated anatomy, on 2026-07-03. Established named construct, not a proposer coinage: *defeasible reasoning* and *default / non-monotonic logic* in logic and AI (Reiter 1980 default logic; Pollock's rebutting vs undercutting defeaters); *rebuttable presumption* and *defeasible right* in law; *prima facie* / *ceteris paribus* duties in philosophy (Ross); *defeasible implicature* in linguistics. Verified absent from the corpus. Manifest: STRONG-MODERATE hold.

**For owner evaluation (keep/cut).** The case FOR is strong: defeasibility is a *canonical, independently named construct* across logic, AI, law, and ethics, verified absent, arrived at by six-input convergence, with a clean and distinctive anatomy — default conclusion + rebutting/undercutting defeaters + non-monotonicity + burden-shifting — that the corpus does not otherwise carry. It fills a real gap: the corpus has `mandatory_vs_default_norms`, but that names only the deontic-*norms* special case (which norms are mandatory vs default/overridable), whereas defeasibility is the general holds-unless-overridden structure of any rule, inference, or factual claim. The RISKS Kurt should weigh: (1) **overlap with `mandatory_vs_default_norms`** — the discriminating claim is *genus vs special case*: defeasibility covers *inferences* (non-monotonic reasoning) and *factual presumptions* (rebuttable presumptions of fact), not only norms; `mandatory_vs_default_norms` is its application to normative rules. Recommend a subsumption edge `defeasibility` → `mandatory_vs_default_norms` (norms as the deontic instance), not a merger. (2) **One head vs two** — whether defeasible *reasoning* (the inference pattern) and defeasible *rules / rights* (the object that gets overridden) are one prime or two; the proposer recommends *one* head, since both share the identical default-plus-defeater + non-monotonicity anatomy and differ only in whether the defeated item is an inference or an object. Sharp keep/cut question: *is "holds by default, overridable by a sufficient defeater, non-monotonically" a distinct prime, or is it just `mandatory_vs_default_norms` read more broadly?* — the proposer's answer is that the general structure spans inferences and factual presumptions the norms prime cannot reach, so it is the genus.

**At incorporation.** Re-run similarity against `mandatory_vs_default_norms` (the proposed special case), `constraint`, `epistemic_justice`, `uniformitarianism`, and `correspondence_principle` (co-convergent inputs — screen for confusion), any presumption / non-monotonic-inference prime, and `counterfactual_reasoning` (screen: counterfactual reasoning is about what-would-follow under altered premises, distinct from the holds-unless-overridden modal status of an actual conclusion) to confirm no collapse. If kept: draw a subsumption edge `defeasibility` → `mandatory_vs_default_norms` (norms as the deontic case), and add a missing-edge note flagging the genus-vs-special-case relationship explicitly. Resolve the one-head-vs-two question at incorporation (recommend one head covering both defeasible reasoning and defeasible rules/rights). Write a v1 draft and sync the one_liner across v1 and v2 per CLAUDE.md before merge.
