Onus probandi means burden of proof. Generally, a party who alleges an affirmative position has to prove it. It is also a general rule that the onus probandi lies on the party who seeks to support his case by a particular fact of which he is supposed to be cognizant. However, when law presumes the affirmative, onus probandi lies on the party who denies the fact, to prove the negative. For example, law presumes the legitimacy of children born in wedlock. It is the duty of person denying legitimacy to prove the allegation.