fallacy

No true Scotsman

Also called: Appeal to purity

The no true Scotsman fallacy protects a generalization from a counterexample by redefining the group so the counterexample no longer counts.

How it works

Instead of revising the claim, the speaker changes its terms after the fact. The generalization becomes unfalsifiable: any member who behaves differently is declared not a real member.

Examples

"No supporter of our movement would ever use violence." "This one did." "Then he was never a true supporter."

The definition of "supporter" is changed to exclude the inconvenient case.

"Real investors never panic-sell." "Many did last year." "Then they weren't real investors."

The claim survives only because counterexamples are defined away.

How to spot it

"True", "real" or "genuine" added after a counterexample appears.
The group's definition shifts mid-argument.
No case could ever count against the claim.

How to respond

Ask for the definition of the group before discussing examples.
Point out that the claim now cannot be tested.
Suggest a qualified claim, such as "most" or "our stated principles reject".