Article I. On the Nature and Division of Probability

Probability is nothing other than that motive, one or multiple, which, all the circumstances having been inspected, is capable of extorting the assent of a prudent man, yet not in such a way that no fear of error remains with him. Wherefore, when we say this or that proposition is probable, we understand two things, one positive, the other negative. The first is that a proposition of this sort is supported by so great a motive that a prudent man can safely assent to it; but the other is that that motive is not so great that it must necessarily and without any fear of error extort the assent of a prudent man. If the first is lacking, the proposition will be improbable; if the other, it must be called certain.

Probability is divided in many ways, but four divisions are chiefly considered here. Therefore it is:

1st. Intrinsic or extrinsic, according as it rests proximately either upon reasons deduced from the nature of the thing, or upon authority. It is called proximately, because extrinsic probability necessarily supposes intrinsic probability; namely so far does the authority of Doctors have force, insofar as it can or ought to be judged that they were moved to hold some opinion by some grave reason.

2nd. Speculative or practical, according as it regards the action considered in itself, namely without attending to those concrete circumstances which are commonly joined to it, or rather according as these circumstances are well weighed in each particular case.

3rd. Of law or of fact, according as it is concerned with the law itself, or rather some fact.

4th. Greater, equal, or lesser, according as the foundations upon which some probable proposition rests, comparison having been made with its contradictory, appear more solid, equally solid, or less solid. Cf. Lehmkuhl, n. 79.

Things To Be Noted. I. Probability which is everywhere among authors called slight or doubtful is understood as that which is so light that he who assents to it is judged imprudent; for St. Alphonsus says: "Slight probability is not probability, but only a certain false appearance, or empty apprehension of probability." Therefore some authors rightly observe that those two terms, slight probability, are as it were opposite and fighting with one another. Cf. St. Alphonsus, n. 82. - Lacroix, n. 126. - Bouquillon, n. 308.

II. A proposition or sentence which in St. Alphonsus is called most probable is, according to him, that "which rests upon a most grave foundation; wherefore the opposite is judged slightly or doubtfully probable." The same, in the holy Doctor, is the sense of the word more probable, when one of these four adverbs is added to it: notably, certainly, truly, absolutely. Cf. St. Alphonsus, nn. 40 and 54.

III. Some proposition can be more probable and at the same time less safe, and vice versa safer and less probable. The reason is that probability is measured from the weight of the motives, and safety from the absence of danger of harming the law.

IV. An opinion is to be held certainly probable which is asserted by most theologians to be probable; or even which five or six theologians outstanding in probity, judgment, and knowledge absolutely hold as true, unless a certain reason is brought against that opinion; for every probability, however great it may have been, when certainty of the contradictory proposition arrives, is altogether destroyed.

V. That opinion is certainly probable for a learned and upright man which he himself knows he has diligently examined, and experiences himself, without inordinate affection, on account of grave reasons, to find as true or certainly probable.

VI. Concerning the intrinsic probability of law, commonly only the learned and those versed in moral matters can judge, since only they can know that nothing certain is opposed to them. But concerning the extrinsic probability of some opinion, a moderately learned man can well judge, if, rightly understanding the state of the question, he finds this opinion asserted by authors greater than every exception, that is, altogether approved among theologians.

VII. A single author, if he is greater than every exception, and brings forward a reason which others have not examined or have not sufficiently solved, while he himself conveniently solves the reasons of others, can make his opinion certainly probable, although this is contrary to the common opinion.