(b) In other cases one may change the probability of fact into a probability of law by recourse to a probable opinion or argument that under the existing doubt of fact the legislator does not wish the law to oblige. Examples: Titus, who has what is probably lawful food before him, argues with himself that it is not likely that the Church wills to put him to the expense, trouble, and loss of time required to order other food. Fr. Titus, who has probably said Vespers, argues that theologians of authority teach that, when there is a serious reason for thinking one has performed such an obligation, it may be presumed that the Church does not require more.
713. If a case of probability of fact on which obligation hinges cannot be solved by recourse to the expressed or inferred will of the lawgiver, one has no choice but to follow the safer side, for then, though it is probable that a certain thing is a fact, it is not probable that one has a right to act. Example: Sempronius, while hunting, sees an object moving in the bushes. The probabilities are that it is not a human being, but it is not probable that Sempronius has the right to risk homicide by firing at it.
714. Not all Probabilists use the principle of the presumptive will of the lawgiver for all cases of negative doubt; some employ different principles for different kinds of doubt, and sometimes arrive at other decisions than those given in the preceding paragraphs. Thus, they give such rules as the following:
(a) In negative doubt of law regarding the lawfulness of an act, use the principle that law or liberty should be followed according as one or the other is in possession (see 660). Example: He who has only slight reasons for thinking that a law exists, or that it is of grave obligation, or that it extends to his case, etc., may decide against the law. But he who has only slight reasons for thinking that a law has been abrogated, or that a dispensation has been granted, etc., must decide for obligation.
(b) In negative doubt of law regarding the validity of a past act, use the principle _that what was done is to be held as rightly done_. Example: He who has no reasons, or only trifling ones, for thinking that a Sacrament was not administered validly or received validly, should decide for validity.
(c) In negative doubts of fact, use the principles that one should judge according to what usually happens, or that facts must not be taken for granted but must be established, or that presumption favors that which has possession. Examples: If there is no good reason to think that a conscientious person gave consent to a temptation, one may decide for the negative, since that would usually be true. If there is no good reason to think that one has made a vow, one may decide for the negative, since the burden of proof is with the other side. If, in a question about fast and abstinence, it is uncertain whether or not a person has reached twenty-one years, or whether Friday has commenced, the presumption is for the negative, since liberty has been in possession; but if it is uncertain whether a person has reached the age of sixty or whether Friday has ended, the presumption is for the negative, since the law has been in possession.
715. Having discussed the cases to which Probabilism is not extended, we pass on to the cases to which it is applied. Probabilism is used in any and every case where speculative certainty as to what is lawful or unlawful is not had, but where there is only speculative probability against an opposite probability.
(a) Probabilism is used not only in probability of law, but also in probability of fact that can be reduced to probability of law, as was explained above (see 712).
(b) Probabilism is used in probability of law, whether or not the question be about the existence or the cessation of the law. There is probability against existence of law, when one has good reason to think that a law was not made or not promulgated, or that the time when it goes into force has not arrived, or that it does not apply to certain persons or circumstances, etc.; there is probability for cessation of law, when it is certain that a law did exist, but one has good reason to think that it ceased or was abrogated, that one is excused or dispensed from it.
(c) Probabilism is used in probability of law, whether the law in question be natural, divine or human—that is, in every case of law where invincible ignorance is possible (see 319 sqq., 356).