Article I. On the Nature of Probability

- Division of probability. Probability is distinguished:

Intrinsic, or extrinsic, according as it rests either on reasons drawn from the nature of the thing, or on authority ¹. Each can be greater or lesser.

¹ Extrinsic probability supposes intrinsic; namely the authority of Doctors has force insofar as it can or must be judged that they were moved to holding some sentence by some grave reason. Wherefore if perhaps you detect that they have asserted something while relying on a weak or even altogether false foundation, their authority certainly adds no moment to the assertions.

Of law, or of fact, according as it is concerned with the existence of law, or its application, or with some fact.

Speculative, or practical, according as it regards a thing in general, abstracting from circumstances, or an act proximately to be posited ².

² Perhaps you would more clearly call opinion or sentence speculative or practical according as it is or is not ordered to directing an action which here and now in reality must be posited.

Absolute, or relative, according as it appears such to all or to most, or only to one or a few.

Solitary, or comparative, according as it appears such considered in itself alone, or by comparison instituted with the foundations of the contradictory proposition ³.

³ More commonly the Doctors call that absolute which the Author calls solitary; but they are accustomed to call the other relative.

Certain, or doubtful or slight, according as it rests on motives either certainly solid, or only doubtfully solid.

Comparative probability is divided by degrees into three kinds, namely 1. into greater probability, 2. lesser, 3. equal. For any proposition can be more probable, or equally, or less probable than the opposite, according as it rests on more solid, or equally, or less solid motives. - But the more probable opinion itself also admits various degrees: for it is a little or notably more probable, according as its motives are a little or notably graver than the motives of the opposite. It is also called most probable if it enjoys a most grave motive. Most probable, says St. Lig., no. 40, is that which rests on a most grave foundation: wherefore the opposite is considered SLIGHTLY or DOUBTFULLY probable.

Besides, an opinion is safe, or safer, or less safe, according as it simply favors the law, or favors it more or less. Therefore a more probable opinion is not always safer, since greater probability can also stand for liberty.

Various systems concerning probability.

63.^† - 1. Absolute tutiorism, or rigorism, according to which the safer part, that is, the part favoring the law, must always be followed ⁴.

⁴ Namely, these hold that the safer part must be followed unless it is certain that nothing favors the law against liberty; whence according to them it is not licit to act according to the most probable opinion favoring liberty. This doctrine was condemned in the third of the thirty propositions proscribed by Alexander VIII.

Mitigated tutiorism, according to which it is licit only to follow the most probable part favoring liberty.

Probabiliorism, according to which the opinion favoring the law must always be held, unless the opposite is notably more probable ⁵.

⁵ Since according to St. Alphonsus a notably more probable proposition has against itself only a slightly or doubtfully probable one, here rather mitigated tutiorism is being described.

Equiprobabilism, according to which it is not licit to follow the less safe opinion unless it is as probable as the opposite, or nearly as probable ⁶.

⁶ Some have called St. Alphonsus de Ligorio the author of this system. But this neither ought nor can be so understood, as though the holy man at some time receded from the common system of probabilism, which he himself also defended most keenly. See the Dissertation On the Moral System of St. Alphonsus, which was delivered at the Roman College in the solemn restoration of studies in the year 1863, and is available at Rome from Io. Bencivenga.

Probabilism, according to which it is licit to follow a less safe and also less probable opinion, provided that it enjoys true and solid probability.

Laxism, from which it is always lawful to follow opinions even slightly probable; but this system too was condemned, as will be clear below.

Hence, according to the various opinions concerning the value of probability, theologians have obtained various names, so that they are called either rigorists, or mitigated tutiorists, - probabiliorists, - equiprobabilists, - probabilists, - laxists.

Conclusions From What Has Been Said. - 1. Probability from its nature necessarily requires that it exclude certitude, even merely moral certitude; for by this alone can it differ from certitude ⁷.

⁷ Probability cannot be joined at the same time with certitude, both because it differs far from certitude, and because probability which has certitude against itself wholly vanishes.

Hence every probability, however great it may have been, is altogether destroyed when certitude of the contradictory proposition supervenes.

An opinion must certainly be held probable which, the Church tolerating, is common among theologians ⁸, or is taught by St. Thomas and his school, or is asserted as probable by most theologians; or even which five or six theologians outstanding in probity, judgment, and knowledge hold absolutely as true, unless a certain reason against that opinion appears.

⁸ When the common opinion of Theologians stands for some doctrine, we have probability of this kind such that (unless a plainly grave reason observed by no one, or a clear authority, stands in the way) it is rash to resist it. See Canus, On Theological Places, book 8, ch. 4.

That opinion is certainly probable for a learned and upright man which he himself knows he has diligently examined, and which he experiences himself, without inordinate affection, to detect as true or certainly probable ⁹ on account of grave reasons.

⁹ Namely, it is not necessary that he remain without fear concerning the opposite.

Concerning the intrinsic probability of law, commonly only the most learned and those most versed in the moral matter can judge, since they alone can know that nothing certain is opposed to them.

A man moderately learned can judge concerning the extrinsic probability of an opinion, if, rightly understanding the state of the question, he finds it asserted by authors greater than every exception, that is, altogether approved among theologians.

One author, if he is greater than every exception, and brings a reason which others have not examined or have not sufficiently solved, while he himself suitably solves the reasons of others, can make his own opinion certainly probable, although it is contrary to the common opinion.

An uneducated man, hearing from a man whom he himself judges upright, prudent and learned, that some opinion is certainly probable, can hold it as such: for he is not able to weigh the matter per se, nor is any other means at hand for him of knowing extrinsic probability... - Lacroix..., etc.