A matter, the validity of which is at issue in legal proceedings, cannot be set up as a bar [impediment] thereto.

Explanation

(English-origin maxim.) A proceeding whose validity is the very question at issue cannot be pleaded as a bar to the suit challenging it — to allow otherwise would make every void act self-validating.

Where this applies

• A tenant sues to have an eviction declared void, and the landlord cannot use the eviction itself as a defense to block the lawsuit.
• A party challenges the legality of an election result, and the declared winner cannot cite the election result as a bar to the challenge.
• A citizen sues to invalidate a government regulation, and the agency cannot point to the regulation as grounds for dismissing the suit — the very thing being questioned cannot serve as its own shield.