706. It is not sufficient according to the Probabilists that another be certain of the probability of an opinion; but the person who follows the opinion must himself be certain that it is solidly probable.
707. Regarding the kind of authority necessary to make an opinion solidly probable from external evidence, Probabilism teaches:
(a) that absolute probability (that is, such a weight of authority as would appear strong even to the most learned) ought to be estimated by quality rather than quantity—by the learning, prudence, impartiality, and independent study of the authors, rather than by their numbers. If five distinguished moralists arrive by separate study at the same conclusion (i.e., that an opinion is probable), or if one of special reputation in a matter under question supports the probability of an opinion, the argument from authority is strong;
(b) that relative probability (that is, such a weight of authority as suffices for one who is unlearned, such as a child, a halfwit, an uneducated person) is had sufficiently through the word of only one person who is looked up to as a guide or instructor, such as a parent, confessor, or teacher.
708. Probabilism supposes that one regards the opinion one follows as truly probable, and that one is convinced that it is lawful to follow such an opinion. Hence, the system does not apply in certain cases.
(a) It does not apply to cases in which there is no probability on either side—that is, to cases of negative doubt (see 656 sqq.), whether the doubt be of law or of fact.
(b) Probabilism does not apply to cases in which there is only slight or uncertain probability for the less safe side. Example: Caius has heard that a certain novel opinion is defended by a recent author, but he is uncertain of the author’s standing as a theologian, and he realizes that the fact that a man has written a book does not make his ideas solidly probable.
(c) Probabilism does not apply to cases in which there is solid probability for the less safe side, but one doubts whether one can lawfully follow it; for it is always sinful to act with a doubtful conscience (see 641 sqq.). Example: Caius has read in a reliable work of theology that a person in certain circumstances, which are his own, is probably excused from Mass. But the word “probably” makes him uncertain whether he can follow this opinion.
709. For the above-mentioned cases, to which their principle does not apply, Probabilists refer to the rules for a doubtful conscience (see 656 sqq.). The following special rules are given for cases of negative doubt:
(a) If the doubt is one of law and insoluble, one is free to act; for it is a general principle that an act may be considered lawful, as long as there is no serious reason to the contrary. Example: Sempronius goes out into the country on Sunday afternoon. An opportunity to fish presents itself, but Sempronius begins to doubt whether there is or is not a church law against fishing on Sundays. As no argument for either side is known to him, he may act on the general principle that what is not forbidden is lawful.