Nemo iudex in causa sua

Nemo judex in causa sua (also written as nemo [est] judex in sua causa, in propria causa, in re sua or in parte sua) is a Latin brocard that translates as "no one is judge in their own case". Originating from Roman law, it was crystallized into a phrase by Edward Coke in the 17th century and is now widely regarded as a fundamental tenet of natural justice and constitutionalism. It states that no one can judge a case in which they have an interest. In some jurisdictions, the principle is strictly enforced to avoid any appearance of bias, even when there is none: as Lord Chief Justice Hewart laid down in Rex v. Sussex Justices, "Justice must not only be done, but must also be seen to be done".

History
The Latin brocard nemo judex in causa sua has its origins in the Roman legal tradition and is codified within the Corpus Juris Civilis. In 376 AD, an imperial decree established the principle that "no one shall decide his own case or interpret the law for himself" (neminem sibi esse iudicem vel ius sibi dicere debere) (Code 3.5.1). Additionally, the Digest records Julianus's statement that "it is unfair for someone to be the judge of their own affairs" (iniquum est aliquem suae rei iudicem fieri) (Digest 5.1.15-17).

From these Roman sources, the principle has endured into modern times and can be traced in Martin Luther's 1526 work Whether Soldiers, Too, Can Be Saved (Niemand sol sein selbs Richter seyn, meaning "no one should be their own judge"), in Ulrich Zwingli's In Exodum (1527) in Jean Bodin's The Six Books of the Republic (1576) and in Hobbes's Leviathan (1651) ("And seeing every man is presumed to do all things in order to his own benefit, no man is a fit Arbitrator in his own cause").

In the 17th century, the English jurist Edward Coke turned the idea into a phrase when he wrote that "it is a maxim in law aliquis non debet esse iudex in propria causa". Coke used the principle to instruct the king that he could not personally judge a dispute between himself and his subject. Moreover, In the famous Bonham's Case of 1610 Coke ruled that the College of Physicians could not sit as judges in a case to which they were a party, and he was understood also to have affirmed that that the principle could not be overridden by statutory provision.

Since then, the rule against bias has been recognised as a fundamental tenet of natural justice in the common law tradition and a cornerstone of constitutionalism.The maxim has been invoked by the United States Supreme Court in various cases, such as the 1798 case Calder v. Bull ("a law that makes a man a Judge in his own cause [...] is against all reason and justice") and the 1974 case Arnett v. Kennedy ("we might start with a first principle: '[N]o man shall be a judge in his own cause.' Bonham's Case, 8 Co. 114a, 118a, 77 Eng. Rep. 646, 652 (1610)").