What is Aliquis Non Debet Esse Judex In Propria Causa, Quia Non Potest Esse Judex Et Pars?

Legal Definition
A person ought not to be judge in his own cause, because he cannot act as judge and party. Co.. Litt 141; 3 Bl. Comm. 59
-- Black's Law Dictionary
Legal Definition
One ought not to be a judge in his own cause, because one cannot be both a judge and a party to the action.
-- Ballentine's Law Dictionary