Article — Billuart, Summa Summae · from Schola Thomistica
On the nature of probability
[Since in the present question we treat of the value of probable opinion relative to conscience, in practical matters, it must first be said what opinion is, and then what probability is and in how many ways it is diversified.
[Cum agatur in praesenti quaestione de valore opinionis probabilis relative ad conscientiam, in rebus practicis, primum dicendum est quid sit opinio, deinde quid et quotuplex probabilitas.
Opinion in general is assent to one or the other of two propositions, or adherence to one or the other of two contradictories, with fear of the other. It excludes certainty, which so clearly manifests a thing that the opposite appears not merely doubtful, but impossible.
Opinio in genere est assensus in alterutram propositionem, seu adhaesio alterutri e duobus contradictoriis, cum formidine alterius. Excludit certitudinem, quae rem ita clare manifestat, ut oppositum appareat, non tantum dubium, sed impossibile.
Probability is the likelihood which, according as it is greater or less, draws the assent of the mind more or less toward one of two contradictories. Hence a probable opinion or sentiment is a judgment resting upon a weighty motive or foundation, though with fear of the opposite. An improbable opinion, however, is a judgment destitute of a solid motive. Whence it seems that opinion, so long as it does not exceed the limits of probability, however probable it be, does not altogether destroy the likelihood of the contradictory proposition, but only weakens it, according to the solidity of the foundation on which it itself rests.
Probabilitas est verisimilitudo quae, prout est major vel minor, magis vel minus trahit assensum mentis in unum e duobus contradictoriis. Ideo opinio seu sententia probabilis est judicium innixum motivo seu fundamento gravi, licet cum formidine de opposito. Opinio vero improbabilis est judicium motivo solido destitutum. Unde videtur opinionem, quamdiu fines probabilitatis non excedit, quantumvis probabilis sit, non omnino destruere verisimilitudinem propositionis contradictoriae, sed eam tantum extenuare, pro soliditate fundamenti quo ipsa nititur.
1° It is intrinsic, when the reasons establishing it are sought from the very nature, properties, causes and effects of the thing, and also from the inconveniences of the opposite side. It is extrinsic, when it rests upon the authority of weighty doctors who deservedly consent that this sentiment should be held on account of solid reasons, even if those reasons have not been weighed in themselves.
1° Est intrinseca, quando rationes eam statuentes petuntur ex ipsius rei natura, proprietatibus, causis, effectibus, et etiam ex inconvenientibus partis oppositae. Est extrinseca, cum nititur authoritate doctorum gravium, qui hanc sententiam merito consentur tenere propter rationes solidas, etsi rationes istae non fuerint in se trutinata.
2° It is of law or of fact, according as it turns upon the existence of a right or law and its application, or upon some fact.
2° Est juris vel facti, prout circa juris aut legis existentiam et ejus applicationem, vel circa aliquod factum versatur.
3° It is speculative or practical, according as it regards the question in itself, abstracting from every act here and now to be posited, or an act to be posited presently.
3° Est speculativa vel practica, secundum quod quaestionem in se, abstrahendo ab omni actu hic et nunc ponendo, vel actum proxime ponendum respicit.
4° It is absolute, if on account of the validity of its reasons it is admitted by all or by most; or relative, when only one man or a few are determined by arguments that do not draw common assent.
4° Est absoluta, si propter validitatem rationum ab omnibus vel plerisque admittatur, vel relativa, cum unus tantum vel pauci determinantur argumentis quae communem assensum non trahunt.
5° It is solitary or comparative, according as such it appears in itself, without any relation to another, or its reasons are compared with the arguments which contend for the contradictory proposition.
5° Est solitaria vel comparativa, prout talis apparet in se, absque ulla relatione ad aliam, vel ejus rationes cum argumentis quae pugnant pro contradictoria propositione comparantur.
6° Comparative probability is divided by degrees into greater, less and equal: that is, a certain proposition is more probable, or equally or less probable than its opposite, according as it rests upon stronger reasons, or upon reasons equally or less strong.
6° Probabilitas comparativa dividitur per gradus in majorem, minorem et aequalem : id est, propositio quaedam est probabilior, vel aeque aut minus probabilis quam opposita, prout nititur rationibus solidioribus, vel aeque aut minus solidis.
7° A more probable opinion also admits of degree; for an opinion is more probable, or notably preponderant, when the reasons of the sentiment preponderate; a little or notably more probable than its adversary, when
7° Opinio probabilior gradus etiam admittit; est enim probabilior, vel notabiliter praeponderans, quando rationes sententiae contradictoriae praeponderant. paulo vel notabiliter probabilior quam adversa, quando
8° Most probable likewise is the opinion which rests upon a most weighty foundation. Then the opposite is slightly or doubtfully probable.
8° Probabilissima etiam est opinio quae gravissimo fundamento nititur. Tunc opposita est tenuiter vel dubie probabilis.
9° Moreover, a probable opinion is safe, or safer, or less safe, according as it simply favors the law, or favors it more or less than does its adversary. Therefore a more probable sentiment is not necessarily safer, since a greater probability can also stand for liberty.]
9° Insuper opinio probabilis est tuta, vel tutior, vel minus tuta, secundum quod simpliciter favet legi, vel eidem magis vel minus favet quam adversa. Igitur sententia probabilior non est necessario tutior, siquidem probabilitas major etiam pro libertate stare potest.]