Article — Marc, Institutiones Morales
Article I: On the Nature of Probability
- Definition. Opinion is an assent of the mind toward one part of a contradiction with fear of the other (St. Thomas, I, q. 79, a. 9, ad 4). Insofar as it is assent, opinion differs from doubt, which is a suspension of judgment assenting to neither part. Insofar as it is with fear of the opposite part, it does not signify a departure from the part to which it assents, but a defect of the firmness of science, and thus it differs from science strictly so called, and from certitude, which is an assent excluding founded fear. See what was said n. 33.
Probable, by force of etymology, signifies the same as what can be proved by reasons. But probability is an appearance of truth, which can arise from various causes and admits various grades.
A probable opinion can be considered both subjectively and objectively. Considered subjectively, it is the assent of the mind given to one part on account of a suitable motive persuading of its truth, yet fallible, and therefore with fear of the opposite part. Considered objectively, it is a proposition which rests on a grave foundation, either intrinsic, of reason, or extrinsic, of authority, which is able to draw to itself the assent of a prudent man, although with fear of the opposite. The things which the doctors commonly dispute concerning probable opinion regard opinion taken objectively.
- Division. 1. By reason of the object, probability is of fact or of law, according as it is concerned with the truth of the thing or with the honesty of the action, as was explained above (n. 32, 2).
By reason of the foundation, probability is distinguished as intrinsic or extrinsic. Intrinsic is that which rests on the weight of reasons drawn from the nature itself of the thing; extrinsic is that which rests on the testimony and authority of doctors.
By reason of use, it is divided into speculative and practical (III, 700, note). Speculative (or remote) probability is the verisimilitude of an opinion considered in itself, insofar as it is an object of knowledge, without any application to work. Practical (or proximate) probability is the verisimilitude of an opinion considered with respect to work, insofar as namely, with all things weighed, it can serve for a certain and practical dictate of conscience (cf. n. 80, 2).
- These things concerning the notion of probability having been premised, it must carefully be noted that probable opinions can be considered both in themselves and relatively or comparatively to the opposite opinions. But since this matter is of great moment, it must be set forth somewhat more fully.