Nemo judex in causa sua (or nemo judex in sua causa) is a Latin phrase that means, literally, no-one should be a judge in his own cause. It is a principle of natural justice that no person can judge a case in which they have an interest. The rule is very strictly applied to any appearance of a possible bias, even if there is actually none: "Justice must not only be done, but must be seen to be done".
May also be called:
nemo judex idoneus in propria causa est
nemo judex in parte sua
nemo judex in re sua
nemo debet esse judex in propria causa
in propria causa nemo judex
The legal effect of a breach of natural justice is normally to stop the proceedings and render any judgment invalid; it should be quashed or appealed, but may be remitted for a valid re-hearing.