(a) in negative doubts when the validity of acts is at stake. Example: Titus is uncertain whether the law requires the age of fourteen for a valid contract of marriage; he is also uncertain whether he is fourteen years old. The doubt of law and of fact does not excuse Titus from the law, if he wishes to marry. He must clear up the doubts, and if necessary he must secure a dispensation. (b) in negative doubts when reasons of charity or justice forbid one to take risks. Example: Caius is uncertain whether he paid Sempronius for work done for him. He is bound to make inquiries about the matter. 711. Probabilism cannot be applied, therefore, when the mental state of the subject is doubt, weakly founded opinion, or practical uncertainty. But, even when one holds an opinion as solidly and certainly probable, one may not follow it as a moral guide, if there is something in the nature of the object or matter itself which forbids this. (a) A probability of law favoring liberty may not be followed in those matters in which some natural, divine or human law requires one to follow the safer side (see cases enumerated above, 678, 661). Example: The following opinions are probable; that instruction regarding the Trinity and the Incarnation is not indispensable for salvation; that rye-bread is valid matter for the Eucharist. But in practice it would be unlawful to take the risk of following these opinions, except in cases of extreme necessity, when nothing else can be done. (b) A probability of fact favoring liberty may not be followed so long as there remains nothing more than probability of fact; for, while the will of the lawgiver may on account of probability of non-obligation change one’s relation to the law from obligation to non-obligation, it does not change facts. Examples: On Friday Titus doubts whether a dish before him is meat or fish; probably it is meat on account of its appearance, probably it is fish on account of its odor. At night Fr. Caius is much fatigued, and doubts whether he has said Vespers. Probably he did not, because he cannot recall what feast will be celebrated tomorrow; probably he did, because he remembers having said Compline. 712. For probabilities of fact, to which as such their system does not apply, Probabilists offer the following solutions: