657. Reflex principles by the aid of which a negative doubt may be solved, when the question is about the existence or non-existence of some fact connected with obligation, are the following: (a) If the fact at issue is one about which presumption may be had from general or personal experience, the doubt may be settled by the principle: “In uncertainty decide according to what usually happens.” Examples: Titus is uncertain whether his boy of seven years has the use of reason and is bound to go to Mass. As a rule, children attain discretion at the age of seven; and hence Titus should take his boy to Mass. Fr. Caius is uncertain whether he has said Terce. His experience is that such uncertainties on his part have always been baseless in the past; hence, he may consider that he has said Terce as usual. (b) If the fact at issue is one about which no presumption is afforded, either from general or personal experience, recourse may be had to the principle: “A fact should not be taken for granted, but must be proved.” Examples: Sempronia doubts whether her practice of saying the Rosary daily was the result of a vow; but, as there is no proof or circumstantial evidence of a vow, it may be held that her practice originated in a resolution. Caius, a stranger, claims that Titus owes him for an unpaid debt of his father. Titus knows nothing of the alleged debt, and the only substantiation for its existence is the word of the stranger. Titus is not obliged to pay. 658. Presumption of a fact is of three kinds according to Weight: (a) Violent presumption is based on indications so significant or numerous that it leaves only slight room for evasion. This kind of presumption suffices, but is not essential in solving doubts. Example: Caius has no direct proof or disproof that he paid Titus in a certain business transaction, because all the papers have been lost. But he remembers distinctly that he drew the money and went personally to the office of Titus on the day payment was to be made, and that the latter, up to the time of his death several months later, always acted as if full settlement had been made. (b) Strong presumption is based on circumstances or signs so moving that they permit one to infer a fact as being their natural or usual accompaniment or result. This kind of presumption suffices in solving the doubts we are considering. Example: If Caius, spoken of above, has no individual recollection of any circumstances bearing on the payment of his debt to Titus, but knows that it was his invariable custom to pay all his debts promptly, the presumption that he paid this debt is strong.