What it is
Redefining a category mid-argument to exclude any counterexample.
In the wild
No real engineer writes code like that.
How to answer it
Fix the definition before looking at examples, and hold it fixed after.
Values and premises
No true Scotsman
Also called Appeal to purity
In depth
Origin and naming
Philosopher Antony Flew described the move in God & Philosophy (1966) and returned to it in Thinking About Thinking (1975), where his example became the name: the Scot who reads of a savage crime in Aberdeen and, rather than concede, shifts to 'no true Scotsman would do such a thing'.
More places it shows up
A member of the coding forum says professional developers never use tabs; shown a celebrated engineer who does, they reply that she 'isn't really a professional in the craft sense'.
When the peaceful-demonstration story includes a few broken windows, the organizer insists those 'weren't actual marchers' — before anyone has asked who they were.
A football fan claims no true supporter would leave the stadium early; the empty seats at minute 85 are reclassified as tourists.
In a book club, a reader defends the genre against a bad review: the disappointing novel 'isn't real science fiction anyway'.
How to spot it
- A counterexample appears and the category shrinks on the spot to stay safe.
- Notice the adjective doing the rescue: 'real', 'true', 'genuine', 'authentic' — inserted only after the fact.
- Ask for the membership criteria stated before the example, and the definition has already been quietly rewritten.