nēmō dēbet esse jūdex in propriā causā

Meaning

1. New Latin“No one ought to be the judge in his own trial”: one should not take the law into one’s own hands.
   Ad tertium, cum dicunt, quòd illi, à quibus abstulerunt, fortè habent plus de suo, dicitur, quòd nemo debet esse iudex in propria causa.To the third [point], when they say that those from whom they have stolen perhaps have more than themselves, it is said that no one ought to be the judge in his own trial.1588, Guillaume Pepin, Elucidatio in Confiteor […] , page 350:
   Deberet autem occisio fieri in continenti, seu flagranti: nam ex intervallo, si fur jam coepisset quietè possidere, non liceret; quia tunc non haberet rationem justae defensionis; sed tunc sunt adhibenda Iuris remedia, & auctoritas publica, quia nemo debet esse Iudex in propria causa.The killing would have to have been done in a restrained manner, or in passion: for, from a distance, if the thief had at that point begun to take possession quietly, it would not be lawful; for there would then be no reason of just defence; but there should then be recourse to the remedies of the Law, and to the public authority, for no one ought to be the judge in his own trial.1671, Christoph Haunold, Controversiarum de Justitia et Jure Privatorum Universo […] , volume 1, page 193:

Origins

Ultimately from variations on the Code of Justinian, 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.