No one ought to be a witness in his own cause.

Explanation

A man's testimony in his own cause is inherently suspect because self-interest taints credibility — cite this to challenge the weight given to interested-party testimony, or to argue that a party's uncorroborated self-serving declarations should not carry the day.

Where this applies

• The plaintiff is the only witness to their own claim and nobody else can back them up.
• Someone is testifying about events where they have a financial interest in the outcome.
• A party wants to be both the accuser and the main witness in their own case.

See also

• 9.1.10 No one alleging his own wrongdoing is to be heard as a witness.
• 9.2.5 In court, no one is believed unless under oath.