I need to find his short article on probabilism. Edit. Looks like that article is no longer available.
From his review of Morality of Everyday Life:
But is the speculative presentation of general principles by St. Alphonsus the same as that of St. Thomas Aquinas? Apparently not, according to Fr. Pinckaers...
Another error that is more to the point regards the characterization of Saint Alphonsus’ moral teaching as “probabilism.” Quite precisely, his theory is called “aequiprobabilism.” This school of casuistry holds that the opinion favoring liberty over law may be followed if it is intrinsically probable, all things being equal. This last condition means that, in cases where there is question of the cessation of a law that has already been in force, the opinion favors the law even if the other opinion has probability, but, when there is question of the law having yet come into force, the opinion favors liberty. The simple probabilist holds that any truly probable opinion may be followed, even if an opposing opinion may be more probable. An aequiprobabilist holds the same view but gives greater weight to laws already presumably in force. In casuistic practice, however, these views are merely useful for persuading the penitent, because the confessor may not impose his theory’s resolution of the moral case in question on the penitent, if there exists another view not condemned by authority. In reality, the only two systems of moral evaluation condemned have been rigorism (as in the case of the Jansenists) and laxism (as in the case of some Jesuits), so all the others are practically probable and certainly licit. The Q.E.D. is that the probabilist view wins out, if the penitent wants it to and the confessor keeps within the bounds of his authority. The Thomist Dominic Pruemmer explains in his classic Vademecum:
If one prescinds from rigorism and laxism, each of the systems described is tolerated by the Church, and so the confessor has no right to impose his system on the penitent, or strictly require anything of the penitent which he is not bound to do according to the approach of another legitimate system. Thus the confessor may prudently counsel safer or more probable opinions, but he cannot strictly impose them (that is, in preference to merely probable ones). In practice let him choose those opinions which, considering all the circumstances, he foresees will produce the best fruit for the spiritual health of the penitent.
Thus, Dr. Fleming’s intuition is fundamentally sound: Probabilism, which favors liberty because of a respect for circumstances, is the default system of classical Roman Catholic casuistry. Even so, it is not Roman Catholic casuistry he is promoting but rather a return to any casuistic system at all (including Talmudic or Caroline) within the traditions that have made up our society, for such systems by their very nature harmonize with life as it is actually lived and use morality to preserve and strengthen rather than to break down and overturn ties of blood and soil and common endeavor. Apart from those few things one may never do under any circumstances—such as blaspheme, murder the innocent, commit unnatural acts, or steal from a man poorer than oneself—it is almost impossible to indicate specific acts that must always be done regardless of circumstances. For this reason, then, there must be casuistry, since the possibilities for doing good are literally infinite.
For every manual of casuistry, there needs to be a speculative presentation of general principles. Otherwise, the ethics inculcated may be merely a kind of positivistic integralism, a “this is the way its always been, so don’t ask questions” attitude, unable to defend itself from the critical and revolutionary spirit. This companion volume to The Morality of Everyday Life has yet to be written, but here the reviewer dares to present a suggestion as to what its overarching, unifying insight should be. The exposition of nominalism in the sixth chapter points in the direction of the deepest level of moral reasoning. Whereas Dr. Fleming’s interpretation and application of the genesis of the notion of individualism is not one to which I would subscribe, this is the one place in the book where he brushes up against the larger philosophical issue underlying any account of the morality of human acts and transcending any given instance of moral reasoning.