nemo debet esse judex in propria causa

Etymology
Ultimately from variations on the  of, 3.5.0: “Ne quis in sua causa iudicet vel sibi ius dicat” (“Let no one pass judgement in his own cause, nor speak the law unto himself”). Current as nemo debet esse judex in propria causa and similar forms as a maxim by the 16th–17th centuries in Continental Europe with reference to vigilantism. Applied literally to interested judges in later common law.

Proverb

 * 1)  “No one ought to be the judge in his own trial”: one should not take the law into one’s own hands.