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No True Scotsman

Also Known As: Appeal to Purity
Informal Fallacy ID: no_true_scotsman

Definition

No True Scotsman is an ad hoc rescue of a universal claim by redefining the group in question to exclude counterexamples. When faced with evidence that contradicts a generalization, the arguer modifies the definition rather than accepting the falsification. It transforms an empirical claim into a tautology by making group membership contingent on the very property being asserted.

Examples

"No real programmer uses tabs for indentation." "But John uses tabs, and he's been coding for 20 years." "Well, no true programmer would do that -- he's just a hobbyist."

'No real conservative would ever support raising taxes.' 'But Senator Collins is a conservative and she voted for the tax increase.' 'Well, she's not a true conservative then — she's just a RINO.'

'No genuine fitness enthusiast skips leg day.' 'My friend Marcus is really into fitness and he focuses only on upper-body training.' 'Then he's obviously not a real fitness enthusiast — just a casual gym-goer.'

Formal Logic Pattern
FOL Pattern
The First-Order Logic formula representing this reasoning pattern's logical structure.
FOL (First-Order Logic) uses quantifiers (∀ = for all, ∃ = there exists), connectives (∧ = and, ∨ = or, ⇒ = implies, ¬ = not), and predicates to capture the essential form of a reasoning pattern. For example, the Ad Hominem fallacy: Person(x) ∧ HasFlaw(x) ⇒ Invalid(Claim(x)). These patterns allow automated verification of logical validity.

∀x(Scotsman(x) ⇒ P(x)); ¬P(a) ⇒ ¬TrueScotsman(a)
Formal Verification:
Formal Verification
Checks whether a reasoning pattern is logically valid or invalid using an automated theorem prover.
Formal verification uses an SMT (Satisfiability Modulo Theories) solver — specifically Z3 — to mathematically check whether an argument's logical structure is valid. Each reasoning pattern is translated into First-Order Logic and tested: Can the premises be true while the conclusion is false? If yes, it's formally invalid. If no, it's formally valid. Many real-world patterns (analogies, heuristics) cannot be fully captured in formal logic — these are marked as not formally decidable, which doesn't mean they're wrong.
Not formally decidable

Verification Steps
Verification Steps
Binary yes/no questions that an AI must answer to detect a reasoning pattern in a text.
Each of the 452 aspects has verification steps — simple yes/no questions designed to systematically detect whether a pattern appears in a text. For ad hominem: "Does the argument attack a person rather than their claim?" For false dichotomy: "Are only two options presented when more exist?" This ensures consistent, reproducible analysis.

Binary (yes/no) questions an LLM must answer to identify this aspect:

  1. 1

    Is a universal generalization made about a group?

    Type: binary
  2. 2

    When confronted with a counter-example, is the group definition altered to exclude it?

    Type: binary
  3. 3

    Is the redefinition ad hoc (created specifically to avoid falsification)?

    Type: binary
Deep Dive
The expandable detail section on each aspect page with examples, psychology, and counter-strategies.
The Deep Dive section provides in-depth information about each aspect: a real-world example showing the pattern in action, an explanation of why it works psychologically, practical advice on how to counter it, alternative names, and links to related aspects.

Hierarchical Context