Burden of proof refers to the obligation of a party to prove his allegations during a trial. Typically, the plaintiff must prove whatever allegations he included in his civil complaint by a preponderance (51% or greater likelihood) of the evidence in order to win his case. The defendant is given the opportunity to submit evidence to rebut (contest) the plaintiff's case. In some issues, such as an affirmative defense, the burden of proof may shift to the defendant. For example, a person denying responsibility by reason of insanity must then come forward with evidence of their mental condition.