Nemo judex in parte sua – the legal phrase comes from the Latin language. Literally it means “nobody can judge on their own case. It represents a fundamental principle of Law in all jurisdictions, where it proclaims that no person can deliver justice on a case in which he or she participates as a party (i.e. having personal rights and benefits). For example, if you have interest in receipt of funds in litigation procedure, you cannot be appointed as judge who to decide on this lawsuit. This is why magistrates, who are personally interested in the result of a court procedure, are to be withdrawn from it and substituted by non-interested ones. This also applies in situations where the judge is a witness of a wrongful action or crime, which relates to the lawsuit they are deciding on.
May also be called “nemo judex in sua causa” or “nemo debet esse judex in propria causa”.