The name applied by moralists to the system which holds that whenever a doubt exists concerning the licitness or illicitness of an act, and the doubt cannot be resolved, one is permitted to follow a solidly probable opinion in favor of liberty, even if the opposite opinion in favor of the law is more probable.
The motto of probabilism is Lex dubia non obligat, that is, "A doubtful law does not bind."
A doubtful law does not bind, the defenders of probabilism maintain, because a doubtful law has no claim on the will, which by its very nature is in possession of liberty.
All authorities agree that when there is question of a certain result which must by all means be secured, for example, the validity of a Sacrament, then a merely probable opinion may not be invoked, but that one must be followed which is absolutely safe and certain.
Probabilism was originally given systematic form by Bartholomew Medina (1527-1581).
His doctrine met with favor among a large number of moralists.
Later, however, the Jansenists raised a fierce storm of opposition against it.
The dispute went on for about a century.
Finally it was ended by the thorough and scholarly treatment of the question by Saint Alphonsus.
Today probabilism, either in the form of requiprobabilism or absqlute though moderate probabilism, is the doctrine commonly held by moral theologians.
New Catholic Dictionary