A field guide to the ways reasoning breaks
Attacks on the source
Also called Pre-emptive discrediting
Discrediting a person in advance so that whatever they say is received as suspect.
Before he speaks, remember he has been wrong before.
Name the move out loud, then insist the argument be heard on its own terms.
The term was fixed in its fallacy sense by John Henry Newman in 'Apologia Pro Vita Sua' (1864). Rebutting Charles Kingsley, Newman accused him of trying 'to poison by anticipation the public mind against me', adding: 'This I call poisoning the wells.' The image is older than the label — a contaminated well keeps poisoning every bucket drawn from it.
Before the candidate's first town hall, an opponent runs an ad that only recaps her old business failure, so every word she says on the economy arrives pre-dismissed.
A coworker introduces the new analyst to the team by adding, 'He means well, even if his last report had to be retracted.'
A subreddit pins a comment warning that the author of an incoming long post 'is known for conspiracy theories' before anyone has read a sentence of it.
Opening the parent-teacher meeting, a parent says, 'We all remember what happened with the last field trip,' and the teacher's entire proposal is heard through that frame.