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"Affirmati Non Neganti Incumbit Probatio" is a Latin maxim that means “the burden of proof is upon him who affirms - not on him who denies."
Generally it is the duty of the person who asserts something to produce evidence in order to prove it. It is the duty of the party affirming something to submit sufficient evidence on an issue in order to avoid dismissal of the claim. In a criminal trial the burden of proof required of the prosecutor is to prove the guilt of the accused "beyond a reasonable doubt". In a civil trial, the plaintiff must prove their case by a preponderance of the evidence, which translates to a 51% likelihood that all the facts necessary to win a judgment as presented are probably true.
On some issues, the burden of proof may shift to the defendant, such as when the defendant raises a defense which requires affirmative proof by the defendant to defeat plaintiff's claim. If at the close of the plaintiff's presentation he/she has not produced sufficient evidence on a necessary fact the case may be dismissed without the defendant having to put on any evidence.