No True Scotsman
Redefining a group to exclude counterexamples.
Examples
- She says she's an environmentalist but she flies in airplanes. No genuine environmentalist would have such a carbon footprint.
- That wasn't real socialism. A true socialist system has never been tried.
- Person A: No web developer would use PHP. Person B: Lots of web sites are developed with PHP. Person A: Well they aren't real web developers.