Chapter — Slater, Manual of Moral Theology
Chapter III. Different Species of Sins
THE Council of Trent teaches1 that all Catholics are bound by divine law to confess to a priest all the mortal sins into which they may have fallen after baptism. A confession of sin in general terms will not suffice, but as far as is possible the confession must be integral—that is, each and every mortal sin must be confessed according to number and species. The confessor, then, must know when this duty is sufficiently fulfilled, or he must know how to distinguish the different species of sin. To enable him to do this, theologians have formulated three rules, of which sometimes one, sometimes another, is more serviceable for determining the species of any particular sin.
p. i, 89
Rule I.—Sins differ specifically according as their formal object differs. This rule is merely an application of the universal principle that acts are specified by their formal objects. Sins are bad human acts, and so, as we saw when treating of human acts, their formal object gives them their specific moral quality. The formal, not the material, object specifies the sin—that is, the object, inasmuch as it is morally wrong, causes the will which tends to it to be a vicious will in a certain definite way. And so adultery is a specifically different sin from fornication, because in the former case right order is doubly violated in a way that does not belong to fornication.
p. i, 89
Rule II.—Sins are specifically different according as they are opposed to specifically different virtues. The reason of this rule is fundamentally the same as that of the former, for virtues are specifically distinguished according to their acts, and acts are specifically distinguished according to their formal objects. And so, inasmuch as charity is a different virtue from justice, hatred, as being opposed to charity, is a specifically distinct sin from theft or detraction, which are against justice.
p. i, 89
Rule III.—Those sins are specifically distinct which are transgressions of formally distinct laws.
Laws, however, are formally distinct not because they are made by different authorities; the same sin of theft is against the natural, divine, ecclesiastical, and civil law. But when the motives of two laws are different, and the legislators wished to impose on their subjects the obligation of the special motive which they had in view, the laws will be specifically different, and sins against them will be specifically distinct. Thus the Church commands her children to abstain from flesh meat on Fridays, in order to exercise themselves in the virtue of temperance by curbing their appetite; she forbids anyone to receive Holy Communion who has not been fasting from midnight, out of reverence to the Blessed Sacrament; these two laws, then, are formally different laws, and violations of them are specifically distinct sins. Sometimes the Church, in forbidding an action, does not choose to clothe her precept with the obligation of the motive which induced her to make the law, and then violations of the law will be simply sins of disobedience. Thus bad books are frequently forbidden with a view to safeguarding the faith, but one who reads such books unlawfully does not thereby and necessarily sin against the faith.
p. i, 89–90