In Catholic moral theology, probabilism provides a way of answering the question about what to do when one does not know what to do. Probabilism proposes that one can follow an authoritative opinion regarding whether an act may be performed morally, even though the opposite opinion is more probable. (An opinion is probable when, because of intrinsic or extrinsic arguments, it is able to gain the assent of many prudent men.) It was first formulated in 1577 by Bartholomew Medina, OP, who taught at Salamanca.
Probabilism is a way of approaching difficult matters of conscience. In such cases, according to probabilism, one may safely follow a doctrine approved by a recognized Doctor of the Church, even if the opposite opinion is supported by—or "more probable" as judged by—other criteria, such as those of science or other authoritative sources.
A more radical view, "minus probabilissimus", holds that an action is permissible if a single opinion allowing that action is available, even if the overwhelming weight of opinion proscribes it. This view was advanced by the Spanish theologian Bartolomé de Medina (1527–1581) and defended by many Jesuits such as Luis Molina (1528–1581). It was heavily criticised by Blaise Pascal in his Provincial Letters as leading to moral laxity.
Opposed to probabilism are:
probabiliorism (Latin probabilior, "more likely"), which holds that when there is a preponderance of evidence on one side of a controversy one is obliged to follow that side
tutiorism (Latin tutior, "safer"), which holds that in case of doubt one must take the morally safer side
After its formulation by the Dominican Medina in Salamanca late in the 16th century, probabilism was widely held by respected Catholic theologians, including many Jesuits and Dominicans, for the next century. Jesuits such as Gabriel Vásquez further developed probabilism, distinguishing intrinsic, argument-based probabilism and extrinsic, authority-based probabilism. Abuses of probabilism led to moral laxism such as that of Juan Caramuel y Lobkowitz.