Special case: Someone is the only source of factual information of some sort. "I saw that gun in Mary's purse." In such a situation the whole question of accepting or rejecting what is said revolves around the character of the witness. But note, it is _not_ an argument that is involved.
Hypthetical case: You attack the writer of apost toundermine her arguments. She comes back with: Ha; that post was plagiarized from the correpondence of Marx & Engels.
Carrol