- If the target's statements are being used as evidence because no better forms of evidence are available, then the target's credibility is relevant, and an attack on the target's credibility is therefore also relevant/valid.
- If the target's statements are an argument based on other evidence, then an attack on the target's credibility is irrelevant because it does not address the substance of the target's argument. This is a form of association fallacy.
It is also reasonable (if not directly logical) to dismiss a target's arguments without examining them if the target past history of inventing facts or using bad logic in the deriving of conclusions from agreed facts (as in trolling and the Gish gallop).
Particularly, relationship to the parties in a conflict, or financial interest in the outcome.
Prior statements or comments in conflict with current testimony.
A demonstration of "bad" character as regards truthfulness.
Capacity to witness or judge evidence.
Inconsistencies within the present discussion.
Especially discussion or testimony which includes facts not included in a prepared written statement or summary. The key is to show that the subject is aware of the information and is aware that it is important enough to be included.
Under the United States Federal Rules of Evidence, any party may attack the credibility of any witness. The "BICCC" mnemonic guides evidence: bias, inconsistent statement, character, competency, and contradiction.
- Wikipedia: Witness impeachment
- Credibility and Impeachment, presentation by John Zavitsanos, 2007, University of Houston Law Foundation.