Witnesses cannot testify to a negative; they must testify to an affirmative.

Explanation

(English-origin maxim.) Testimony must be affirmative — a witness can testify to what he saw, heard, or did, but not to what did not happen. Negative proof is inherently weaker than positive proof.

Where this applies

• A witness is asked to swear that an event never happened anywhere at any time.
• Testimony is offered to prove a thing did not occur, rather than to affirm what was actually observed.