Card 17 of 24 · Presumption
Redefinitio ad hoc
Redefining the club after someone proves you wrong.
When a counterexample appears, you change the definition.
Example
“No real mountain local drives that vehicle.”
“What about Tom?”
“Well, no true mountain local does.”
No classical name; coined by Antony Flew (1971). It is an ad hoc rescue that makes a universal claim unfalsifiable by redefining its subject.
Diagnosis: The definition of K is quietly changed to exclude the counterexample. The original claim is preserved only by becoming true by definition — and therefore empty.
“No true Christian would ever do something so evil.” Scripture itself treats believers as capable of grave sin (David, Peter). Better: say the act contradicts Christian teaching.
Not every boundary is fallacious: confessional standards define membership in advance. The fallacy is redefining only after a counterexample appears.
“No real philosopher accepts libertarian free will.” Many do; the claim should be argued, not secured by gatekeeping.
Trail recovery
Ask for the definition in advance. If a group has clear, stated criteria, apply them consistently; if not, revise the universal claim to “most” or “ideally.”
Try saying: “What’s the definition we’re working with — and did we have it before this example came up?”
Keep questioning. Keep exploring.