No True Scotsman

Definition

Let:

• $C$ : A category
• $x$ : A counterexample that belongs to $C$
• $P$ : A property claimed to hold for all members of $C$

Then an argument commits the No True Scotsman fallacy if and only if it asserts:

$$x\in C\text{ and }\neg P(x)⟹x\notin C\text{ (by redefining }C\text{ to exclude }x)$$

Interpretation

The category is redefined ad hoc to exclude counterexamples, making the claim unfalsifiable.