Nemo judex in causa sua or Nemo Judex In Parte Sua

Latin: no person can judge a case in which he or she is party or in which he/she has an interest.

Latin, and a fundamental principle of natural justice which states that no person can judge a case in which he or she is party or in which he/she has an interest.

Also known as:

• nemo judex in sua causa; or
• nemo debet esse judex in propria causa.

The maxim crystalized in British-tradition common law in the case Frome United Breweries Co. v Bath, in which British's highest legal officer, called the "Lord Chancellor" (LC), made a decision favourable to a canal company.

At the time, unbeknownst to the parties to the litigation, the LC was a shareholder in the canal company and had not told the litigants. The LC's decision was set aside because of the nemo judex maxim.

"The maxim that no man is to be judge in his own case should be held sacred. And that is not to be confined to a cause in which he is a party, but applies to a cause in which he has an interest....

"This will be a lesson to all ... tribunals to take care, not only that in their decrees they are not influenced by their personal interests, but to avoid the appearance of labouring under such an influence."

In Canada, the Supreme Court had occasion to reflect on the maxim in Brosseau v Alberta Securities Commission, Justice l'Heureux-Dubé:

"The maxim nemo judex in causa sua debet esse underlies the doctrine of reasonable apprehension of bias.

"It translates into the principle that no one ought to be a judge in his own cause. In this case, it is contended that the Chairman, in acting as both investigator and adjudicator in the same case, created a reasonable apprehension of bias.

"As a general principle, this is not permitted in law because the taint of bias would destroy the integrity of proceedings conducted in such a manner."

See also the related legal definition of recusation.

One early case was Wright v Crump in which the Mayor of Hereford, England, circa 1790,  claimed title to a local house. So he arranged with a friend to lease the real estate to him and then that friend brought a legal action for ejectment of the occupants before, well, Lord Mayor himself, who found for the claimant. The occupants appealed the decision to the Court of King's Bench and the Mayor, sentenced to a term of imprisonment.