ARTICLE I: NOTION OF PROBABILITY

52. • An opinion is called probable which rests upon a grave foundation, whether intrinsic, of reason, or extrinsic, of authority, so that it is able to draw to itself the assent of a prudent man, although with fear of error.

Probability, therefore, 1° adds something to opinion, namely a solid motive, but does not take away fear, and that a prudent fear, lest the contradictory be true. - 2° Between a probable and a certain opinion this distinction intervenes, that the latter excludes every prudent fear of falsity, so that the opposite sentence is reputed altogether improbable.²

Various species of probability. Probability is distinguished in various ways:

1° By reason of the object, one probability is speculative or of fact, which is concerned with the truth of an opinion, for example, whether to paint on a feast day is a servile or liberal work; whether a contract entered upon with such a condition is usurious or not; whether a sacrament conferred with such matter is valid or null. - Another is practical or of law, which is concerned with the licit use of a probable opinion, or with the formal honesty of an action, for example, whether it is licit to paint on a feast day; whether to enter into a contract with such a condition; whether to confer a sacrament with such matter. Cf. n. 48, 3.

Therefore, as we have already said above, the true must be carefully distinguished from the licit; for a speculative doubt, although mediately and consequently it regards the licit, as is clear from the examples brought forward, nevertheless immediately and principally regards the true; but a practical doubt entirely regards the licit.¹

2° By reason of the foundation: a) one probability is intrinsic, which rests upon a grave reason sought from the very nature of the thing; another is extrinsic, which rests upon the authority of grave Doctors; b) an opinion is certainly probable when it rests upon a certainly grave motive; doubtfully probable, when upon a doubtfully grave one; slightly probable, when upon a slight one, namely one of such a kind that it is not able to draw to itself the assent of a prudent man.²

Slight probability is not true probability, as is clear from the definition itself; just as slight fortitude or slight skill cannot be called true fortitude and true skill, but rather must be called weakness and lack of skill. Therefore it is nearly equivalent to possibility, which is not to be esteemed, because little is reputed as nothing, and consequently the slightly probable is equated with the improbable.³

3° By reason of degree, it is distinguished as solitary, sole, or not impeded, to which no probable opinion or reason is opposed; and comparative, to which an equally, less, or more probable opinion is opposed, according as this latter rests upon reasons equally, less, or more solid than the opposite.

Moreover, a more probable opinion is one thing when it is a little more probable, and this is called by St. Alphonsus that which rests upon a foundation a little graver than the contradictory, but at the same time with a prudent fear of the opposite, so that the contradictory also is judged probable. Therefore two opinions of this kind are almost equally probable; but little is reputed as nothing, since a small excess is fallacious. Another is evidently and notably more probable, whose foundations are certainly graver; or most probable, whose foundations are most grave, so that the sentence opposed to these is judged doubtfully or slightly probable. The notably more probable and the most probable are practically equated, because they differ little from each other.¹

Note. - The word most probable can be used in a threefold sense; for it signifies either 1° the most probable of all opinions in general; or 2° the most probable of the opinions which in a particular question are subjected to examination; or 3° the most probable in itself and absolutely taken (very probable). In this third sense it is regularly employed by St. Alphonsus; but the Holy Doctor very rarely uses this ambiguous word in weighing sentences.

53. Comparison of the probability of opposed opinions, according to the mind of St. Alphonsus. - After we have reviewed the various grades of probability, it will not be beside the matter to append a schema by which the reader will perceive at one glance how, according to St. Alphonsus, diverse opinions are opposed to one another. They are opposed thus:

1° To a true sentence, - morally certain: -

An improbable opinion (altogether).

2° To an opinion more true, - most probable, most grave, - more probable (notably, certainly, truly, absolutely): -

Not solidly probable (slightly, doubtfully).

3° To an opinion more probable (a little, doubtfully), - very probable, - probable (truly, certainly, gravely, solidly, absolutely): -

Probable (equally, almost equally, sufficiently).²

54. • It must be noted in this place that probability and the safety of opinions differ greatly. Probability is sought from the appearance of truth; safety, however, from the removal of the danger of sinning; but according as it removes that danger either altogether or more or less, the opinion is called safe, safer, or less safe. Moreover, the danger of material sin and the danger of formal sin can be considered here; and by this reason an opinion less safe with respect to the danger of material sin (as is generally considered in this question) can be altogether safe with respect to the danger of formal sin, which alone is imputed unto fault, as we shall prove below. Finally, although the opinion which stands for the law is safer, but that which stands for liberty less safe, it is clear that the more probable opinion is not always the safer; since the greater probability can be in favor of liberty.