A distinctive innovation of early modern casuistry was probabilism. Originally advanced by a Dominican theologian, Bartolomé de Medina (1527 or 1528–1580), the theory was later taken up by an assortment of Jesuit theologians. Put briefly, probabilism states that in a case of conscience, provided that both options for a course of action are "probable"—that is, they can be justified by right reason, good argument, and sound authority—and that one alternative is more probable than the other, one is permitted to choose the "least probable" (minus probabilis) alternative and is not required to act on the more probable option. Although upheld by many luminaries of early modern Scholastic theology, probabilism failed to convince the vast majority of sixteenth-and seventeenth-century thinkers that it could avoid laxism. Later confections of probabilism, especially those developed by the Cisterican Juan Caramuel Lobkowitz (1606–1682) and the Theatine cleric Antonino Diana (1586–1663), were held to be reprehensible theories that served to endow immoral acts with the token appearance of morality.
The close association of the Jesuit order with probabilism and a residual anti-Jesuit feeling in many quarters of Catholicism and Protestantism conspired to make the terms Jesuit, probabilist, and casuist synonymous in the European mind from the mid-seventeenth century onward. It was, however, powerful critiques of Jesuit moral theology that did most to alter the fortunes of casuistry and condemn it to years of decline. One of the best-known broadsides was advanced by Blaise Pascal (1623–1662), whose merciless Les lettres provinciales (1656–1657) dealt a near fatal blow to casuistry. Pascal's brilliant yet highly rhetorical tirade against the Jesuits sought to expose the putative inconsistencies and errors in their probabilistic method and its application to a wide range of moral questions.
Although Pascal's critique was largely judged to have been a success, it would be wrong to think that the production of works of casuistical ethics or an interest in its issues abruptly terminated after 1657. Even when attacked by an impressive array of detractors, ranging from Dominican supporters of "probabiliorism" (the view that one should always choose the more probable alternative in a case of conscience), Jansensist rigorists, and a hodgepodge of Anglican theologians and Puritan divines to secular moral philosophers, casuistry was still a noticeable feature of the eighteenth-century intellectual landscape, especially in Roman Catholic countries. During a time when European culture and philosophy were being changed as a result of the Enlightenment, the old casuistical methods were similarly refreshed and refashioned by thinkers such as St. Alfonso Maria de' Liguori (1696–1787), who sought to recast theories such as probabilism in order to take account of the preceding century of debate. Liguori's efforts ensured the continuation of vestiges of the casuistical tradition in Roman Catholic moral theology up to the Second Vatican Council (1962–1965).