Casuistry, in the widest sense, is the reasoning which enables a man to decide in a particular case between conflicting, or apparently conflicting, duties. Against casuistry so understood no reasonable objection can be raised. Instances of it may be found in the gospels (Matt. xxii. 17; Luke, xiv. 3), in the epistles of St Paul (e.g. 1 Cor. vii. viii. x.), and in the early fathers. Indeed, St Augustine, in his little works, De Mendacio and Contra Mendacium, may be said to have written casuistical treatises. Gradually, however, casuistry developed within the Catholic Church to an extent which is nothing less than a change in kind, and must radically affect the judgment formed of its merits or demerits. First of all, the ecclesiastical canons became more and more elaborate in their enumeration and classification of grave offences, for which certain definite punishments were due, and, more and more, what had been at first a public discipline was transferred to the court of conscience. Only, it was not the individual conscience which passed sentence in place of the bishop's court. The sins were submitted to the judgment of the priest in secret confession, and for this purpose he was bound to provide himself with a 'penitential book'—i.e. with a summary of the penalties attached to particular sins by the law of the church. These books were in use from the 7th to the 13th century. Another great change occurred in 1215, when the fourth Lateran Council made the confession of every mortal sin to the priest a matter of obligation on all Christians. Then, and not till then, casuistry, in the modern sense, began to be. It is generally known among Roman Catholics as 'moral theology,' and may be defined as the science of the confessional. It professes to decide on the nature of all possible actions, to determine when any particular action is sinful or innocent, to distinguish between binding duty and that which is praiseworthy but not obligatory. It weighs motives and intentions, decides the relative gravity of sins, and the aggravating or extenuating circumstances attending them. Instead of stepping in like the casuistry of the early Christians in doubtful cases, it undertakes to regulate all moral action, covering the whole extent of life and entering into each detail. Instead of giving a list of legal punishments imposed by the church, it anticipates the judgment of God in the world to come. This of course requires an examination of actions in detail, which is often ludicrous or repulsive, always unreasonable. Does a man who steals four shillings commit a mortal sin or only a venial one? in other words, is he liable thereby to the punishment of hell or only of purgatory? Does a man who blasphemes twenty saints at once commit twenty sins, or only one? Are we bound under mortal sin to pray once a month, or only once a year? Such are the questions discussed in every manual of casuistry. They are settled by Scripture, reason, the canons of the church, and the opinion of eminent theologians. A further development dates from the introduction of the system known as Probabilism, first propounded by a Spanish Dominican, Medina (1528–81). It rests upon the theory that the moral law does not bind in cases when it is doubtful, and that it may be considered doubtful if theologians of name have denied that it binds in the particular cases. We have only to consider the endless discussions which these theologians have raised on points of practice, to form some idea of the laxity to which this system leads. Adopted by most of the Jesuits, it was hated 'as a pest of morality' in France, where it was exposed in the Provincial Letters of Pascal, and condemned by the General Assembly of the clergy in 1700. But since the French Revolution it has obtained an exclusive supremacy in the Roman Catholic Church, and at this day no other system is ever thought of.
The name of casuists is legion. The Summa de Casibus Penitentialibus, by St Raymund of Pennafort, is among the earliest works on the subject. He flourished about 1228, and his book was followed by a multitude of others with similar titles. The highest authority was attained by St Alfonso Liguori, who wrote in the middle of the 18th century; and the 'Sacred Penitentiary' at Rome has declared that every confessor may safely follow all his opinions. Modern manuals, chiefly based on Liguori, have been written by Scavini,
Gury (annotated by Ballerini), Lehmkuhl, and others. The objections which Luther made, and which Protestants still feel, to the casuistry described above, are directed against its radical principle and not merely its details. The New Testament, they think, teaches that a man is justified by surrendering himself entirely to God through Christ, and it knows nothing of an elaborate system of law, of which a minimum must be observed as the condition of salvation. They believe that a man's own conscience, enlightened by Christian teaching and example, will generally leave him in little doubt as to his duty, and they object to the constant discussion of moral questions, to the desire to ascertain how little we are bound to do, as ruinous to the moral sense. They deny that the subtleties of a pretended science can ever gauge the amount of sin involved in a particular action. God alone can distinguish between the sudden fall which happens in spite of a man's real character and the sin which reveals his inner corruption. Above all, they object that the so-called science must needs rest in the hands of the priests, and that practically a layman must cease in great measure to have a conscience of his own. A few Protestants—e.g. Jeremy Taylor, and the Puritans Perkins and Amesius—have produced casuistical books, but they deal with cases of special difficulty, and are not in the same sphere as the Roman casuistry. Besides, such Protestant works are now forgotten, and common sense has recognised the fact long ago that any elements of utility in casuistical discussion may be safely relegated to ethics and pastoral theology.