Nemo iudex in causa sua, Latin for no one (is a) judge in (his) own case.
Originally a legal principle pertaining to ancient Roman judiciary, still an important principle of most modern legal systems.
The idea is that no one can decide his own case in the court of law. Even a judge cannot hear a case against himself.
The reason for this is fairly obvious, and it goes beyond the possibility that one might abuse the privilege of judging oneself and ruling in one's own favor. Rather, no matter how objective one may be in judging others, one can never keep the same objectivity towards oneself. Not even if one genuinely wanted to.
While this principle only applies to the judiciary, it is a very good principle to abide by in all aspects of life. That is why, for example, E2 noders cannot ching their own nodes.
It is also important to keep this principle in mind whenever another Latin principle is used, namely Caveat emptor (Let the buyer beware).
For example, recently Dateline NBC (an American TV show of investigative journalism) aired a segment about a mother whose son was born with a genetic predisposition for behaving in a way that might get him in trouble. Not knowing what to do, she consulted with her Pastor, who recommended a Bible based "school" in Texas.
It turned out that the "school" was actually a place of brutal abuse and torture, both physical and psychological. A place that could not pass state licensing requirements, and was forced to close its door until a new Governor of Texas (George W. Bush) decided that the state could not close a Bible based school.
A licensing body was still required but it had to be one created by religious institutions. As it turned out, the chairman of this "school" was one of the key members of the licensing board. In other words, he judged his own skills at running a school and passed himself with flying colors, while he apparently was not qualified (at least according to Dateline story). No doubt he believed that whatever was going on in the "school" was a good thing.
The mother told Dateline that, had she known about what was going on in the "school," she would have never subjected her son to its environment. | 
