Summa summae S. Thomae, Tomus II

[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.

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.

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.

Probability is distinguished in many ways:

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.

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.

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.

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.

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.

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.

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

8° Most probable likewise is the opinion which rests upon a most weighty foundation. Then the opposite is slightly or doubtfully probable.

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.]