in casuistry, a principle of action grounded on the premise that, when one does not know whether an action would be sinful or permissible, he may rely on a “probable opinion” for its permissibility even though a more probable opinion calls it sinful. An opinion is considered probable either if sound, logical arguments can be cited in its favour (intrinsic probability) or if recognized authorities give it support (extrinsic probability).

Formulated in 1577 by Bartolomé de Medina, a Dominican Christian friar of Salamanca, Spain, probabilism was developed by the Jesuits (Jesuit). The Jansenists, who held that in doubtful cases of conscience one should follow the safer view—i.e., against permissibility (tutiorism, rigorism)—attacked the benignity of the Jesuit confessors as leading to laxity of morals. Excesses of probabilism were condemned by Pope Alexander VII (1666, 1667) and more forcefully by Pope Innocent XI (1679).

Probabiliorism, which enjoins following the more probable opinion, was predominant in the 18th century before the formulation of equiprobabilism (either of two equally probable opinions may be followed) by the moral theologian Alfonso Maria de' Liguori, a doctor of the Roman Catholic church.

In a broader context, Carneades, one of the heads of the Platonic Academy (flourished 2nd century BC), was attacked by his fellow Greeks for advocating an intellectual Skepticism that, they argued, rendered man incapable of any action whatsoever. Carneades replied that “probability” (“approvability”) was a practical guide for day-to-day living.