During the controversies between the probabilists and the probabiliorists, the system known as Æquiprobabilism was not clearly brought into prominence. Æquiprobabilism holds that it is not lawful to follow the less safe opinion when the safe opinion is certainly more probable; that it is not lawful to act on the less safe opinion even when it is equally probable with the safe opinion, if the uncertainty regards the cessation of a law; but that if the existence of the law is in question, it is lawful to follow the less safe opinion if it has equal or nearly equal probability with the safe opinion. Many of the moderate probabilists of the sixteenth and seventeenth centuries foreshadowed in their writings the theory to which, in his later-days, St. Alphonsus adhered.
This view gained vigour and persistence from the teaching of Alphonsus Liguori, who began his theological career as a Probabiliorist, subsequently defended probabilism, especially in a treatise entitled Dissertatio scholastico-moralis pro usu moderato opinionis probabilis in concursu probabilioris (1749, 1755), and finally embraced Æquiprobabilism about 1762. In a new dissertation he laid down the two propositions that it is lawful to act on the less safe opinion, when it is equally probable with the safe opinion, and that it is not lawful to follow the less safe opinion when the safe opinion is notably and certainly more probable. In the sixth edition (1767) of his Moral Theology he again expressed these views and indeed towards the end of his life frequently declared that he was not a probabilist.
Probabilists sometimes hold that St. Alphonsus never changed his opinion once he had discarded Probabiliorism for probabilism, though he changed his manner of expressing his view so as to exclude Laxist teaching and to give an indication of what must be regarded as a solidly probable opinion. As a matter of fact, a comparison between the "Moral Theologies" of moderate probabilists and of Æquiprobabilists shows little practical difference between the two systems, so far at least as the uncertainty regards the existence as distinguished from the cessation of a law.
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