459. A law based on presumption is one in which the lawgiver rules for certain cases according to what experience shows in their regard--viz., that such cases are generally dangerous, or indicative of a particular fact. These laws are not of uncertain force, for the cases in which they cease to oblige are few and definite. 460. When a law is based on a presumption of common danger and that danger does not exist in a particular instance, the law nevertheless obliges (Canon 21); for the end of the law is the common good, and if it ceased for an individual whenever its presumption of danger was not true in his case, everyone could persuade himself that the law did not apply to him, and thus the common good would be defeated. Examples: The law against the reading of irreligious books is based on the presumption of common danger of sin, the law against clandestine marriages on the presumption of common danger of fraud; hence, they oblige even in the particular instances where these dangers are absent. Examples of laws based on the presumption of common danger can be found in Canons 199; 409, Sec. 1; 420; 422; 1022; 1028; 1114; 1116; 1138; 1396; 1398. 461. When a law is based on the presumption of a particular fact that usually happens in the cases with which the law is concerned, and the fact in an individual instance did not happen, does the law oblige? (a) In conscience the law does not oblige of itself, because presumptions must yield to the truth; but it may oblige accidentally, if non-observance would cause great public or private harm. Example: The law presumes that a person born and brought up among Catholics has been baptized, and is therefore subject to the church laws. But if, in fact, the person was never baptized, he is not subject to those laws, as long as he remains unbaptized, unless there be some accidental necessity of keeping them, such as the danger of scandal. (b) Before the public authority the law in question does oblige until the non-existence of the fact presumed by the law has been proved in the manner required by law. Example: When parties contract marriage according to the form prescribed by the Church, the presumption is that the contract was valid, and, as long as that presumption is not overcome, the Church will not sanction a new marriage by either of the parties. But if it can be proved in court that threats or violence produced lack of consent, the obligation not to contract a new marriage will terminate before the law.