700. Replies of the Equiprobabilists.—(a) The old (i.e., probabilioristic) principles of Equiprobabilism are not contrary to the new. A more probable opinion not only balances the opposition by its equal arguments, and thus puts away doubt, but it also wins assent by the surplus in its favor, and thus certitude is had. When the two opposites are equally probable, there is a state of true doubt, but certitude is had by recourse to the principles of possession and doubtful law. These principles proper to Equiprobabilism do not favor Probabilism, if one is impartial in one’s use of them, and willing to use them against as well as for liberty.
(b) Equiprobabilism is not more difficult in its application than Probabilism. It does not require that one determine minutely and exactly the greater or equal probability of the arguments for law and for liberty, or that one devote extraordinary diligence to the solution of the problem. All it requires is that one consider the matter seriously, weigh the arguments on both sides impartially, and decide to the best of one’s ability which side appears to be more probable or to have the presumption in its favor.
701. Probabilism.—The meaning of Probabilism can be seen from a comparison with the opposite systems. (a) Unlike Probabiliorism and Equiprobabilism, Probabilism does not require a greater or equal probability, but permits one to follow what is less probable; (b) unlike Laxism, it does not allow one to follow what is only slightly or uncertainly probable, or to apply the system to all cases of doubt.
702. A judgment is probable when it is supported by arguments that make it seem true, although there may remain reasons for doubt. Examples are conclusions based on analogy, on hypothesis, on the opinions of others, or on the calculus of probabilities.
703. Probability is of various kinds. (a) It is absolute or relative, according as the supporting reasons are grave, either when considered alone, or when compared with the objections. Even the Probabiliorists admit that an opinion that is merely probable may be followed, if it is solidly probable and there is no argument against it (see 649). (b) We have solid or slight probability, according as the supporting motives are or are not such as would move, if not convince, a prudent man—that is, a man who shows good judgment in most things. (c) We have certain or uncertain probability, according as a person is sure or not, after reasonable consideration, that the arguments seem valid and the opinion likely. (d) Probability is internal or external, according as the arguments are drawn from the matter at issue itself (i.e., from its nature, properties, causes, effects, etc.) or from the authority of the doctors who have defended an opinion.
704. Relative probability according to logicians remains even when a lesser is compared with a greater probability. (a) If the opposing arguments are drawn from different sources, the more probable does not attack the less probable, and hence does not weaken its probability. Example: An intrinsic argument has more weight than a mere appeal to authority, but it does not attack the opposite argument, and hence does not diminish its probability. (b) If the opposing arguments are drawn from the same source, each one weakens the opposite, since there is direct opposition. But the more probable does not destroy the less probable, since, in spite of the greater appearance of truth on the one side, there still remains room for the possibility that the other side may be true.
705. A moral judgment is solidly probable when the following conditions are present:
(a) For the judgment there must be an intrinsic or extrinsic argument that would be considered weighty by a prudent man. Example: An opinion that has the support of a universally acknowledged authority is strongly probable, whereas, if it has only the support of one obscure writer, it is only slightly probable.
(b) Against the judgment there must be no decisive argument from authority or reason. Example; The judgment that a certain course of action is lawful because St. Alphonsus permits it, is ordinarily solidly probable; it is not probable, however, if the opinion of St. Alphonsus (e.g., that Catholics may act as sponsors in non-Catholic baptisms) has been disallowed by the Church, or if the argument he uses (e.g., that concerning the amount that constitutes grave matter in theft, which reasons from conditions in his day) is not strong.
(c) The arguments for the judgment must retain their probability, if they are set over against the arguments for the opposite. Manifestly, if the arguments are all satisfactorily answered by the opposite side, the judgment based on them ceases to retain the appearance of truth. Probabilism does not require, however, that one determine the relative degrees of probability in opposite opinions.