Article — Merkelbach, Summa Theologiae Moralis
Article IV: On the Division of Mortal and Venial Sin
St. Thomas, I-II, q. 88, a. 2; De Malo, q. 2, a. 8; q. 7, a. 3.
543. Mortal and venial sin are each either from the genus and per se, or per accidens. - Mortal sin is divided into mortal per se or from the genus, and mortal per accidens: the first, according to itself, by reason of itself, from its proper object or matter, from its specific account, can be mortal; the second is not mortal unless on the part of the agent or act a circumstance accedes to the species. Mortal per se is divided into mortal from the whole genus, which from its species is always mortal, and mortal from its own genus, which in its species admits lightness of matter.
Likewise venial sin is divided into venial per se and venial per accidens: the latter by reason of itself and from the object is mortal, and is not venial unless a circumstance accedes on the part of the agent, such as imperfect advertence; the former is venial by reason of itself and from the object, and is divided into venial from its own genus, which according to itself, from its proper object and from its specific account, cannot be mortal, and venial from lightness of matter, which, although from its specific account and proper matter it can be mortal, on account of the small quantity of the matter about which it is concerned, is venial.
Therefore, leaving aside sins which are grave and light per accidens, there remains the celebrated division into mortal sins from the whole genus, mortal sins from their own genus, and venial sins from their own genus.1 This distinction is per se, according to the proper matter and species of sins, whence also the word genus here signifies species.
1 St. Thomas speaks only of mortal and venial from the genus, and does not distinguish mortal sins into mortal from the whole genus and mortal from their own genus, because he regards the species of sin in the concrete. Hence mortal sin from its own genus, if the matter is grave, is mortal from the genus or from the proper species, and venial sin from lightness of matter is in fact not mortal in the concrete species, but venial.
544. Mortal sins from the whole genus are called those which do not have light matter, because in any part of the matter about which they are concerned, the integral account of disorder is preserved.2 They are mortal from the quality of the matter, whence the whole genus is mortal. Such are:
a) sins which are directly against God, the infinite good, or some divine perfection, that is, by which imperfection or defect of perfection is directly imputed to God, whether they are directly opposed to the theological virtues, such as infidelity, heresy, despair, and hatred of God; or are against religion: explicit worship of a creature, that is, idolatry, or divination, or magic at least with an explicit pact with the devil, temptation of God strictly so called, blasphemy, and perjury; or any explicit and formal contempt of God;3
3 Contempt pertains to any virtue: contempt of divine friendship is against charity, contempt of divine worship or of a vow against religion, contempt of life eternal against hope, contempt of a precept or of one commanding as such against obedience, contempt of parents against piety, contempt of a superior against observance, etc. In the present matter the question is of formal contempt of divine friendship, or of worshiping God, or of God commanding, or of God calling to eternal life.
b) sins which, if lightness of matter were given in them, would most greatly allure the human race to sinning, with immense harm to it, such as lust directly and in itself voluntary admitted outside marriage; and direct breaking of the sacramental seal, which would soon turn men away from the sacrament of Penance;
c) sins whose matter is a grave thing consisting in an indivisible, or whose malice in any matter whatever equally repugns to the grave end intended by the law, such as simony of natural right, homicide, duel, delight of lust sought for its own sake outside marriage, which always is directly opposed to the very order and primary end of nature, violation of the fast before communion, omission of Easter communion or of the mixing of water with wine in the Mass, simulation of a sacrament, etc.
545. Mortal sins from their own genus are those which can be mortal or venial according as the proper matter about which they are concerned is grave or light, contains grave or light disorder; whence they are mortal from the quantity of the matter,4 and the genus contains mortal and venial sins. Such are those which are concerned with a finite and divisible good:
4 Yet they are not grave by reason of quantity alone, but insofar as quantity influences and redounds into quality, and carries with itself notable harm or violation of the moral order.
a) sins against the virtue of religion or the precepts of the first table which are not directly against God, but only against worship, such as undue worship of God, temptation of God broadly so called, sacrilege, violation of a vow or promissory oath, an iniquitous oath, violation of the Lord's day, omission or defect in prayer, celebration or hearing of Mass, etc.;
b) sins which injure the rights of others concerning the goods of person, honor, riches; or bring grave detriment to the sinner in intrinsic goods, such as mutilation of the body, drunkenness, detriment to health; likewise sins which are directly against charity toward neighbor, such as hatred and envy; in one word, sins against the precepts of the second table, except homicide and lust voluntary in itself;
c) sins against grave precepts of the Church whose matter is divisible, such as violation of fast or abstinence, simony of ecclesiastical right, reading of a prohibited book, omission in the canonical hours, etc.
546. Venial sins from their own genus are those which, the same matter remaining, are never mortal, because whatever the quantity of the matter about which they are concerned, they do not contain grave disorder. They are venial from the quality of the matter, and the whole genus is venial. Hence they are not mortal unless the malice of another species accedes. Such generally are:
a) sins committed against one's own good, especially against virtues annexed to fortitude or temperance (for example, against magnanimity, magnificence, patience, perseverance, modesty, studiousness, etc.), which, grave harm excluded, consist in an immoderate appetite or excessive use of an honorable or indifferent thing, for example, of food, drink, sleep, laughter, the use of marriage, goods, money, worship, clothing, praise, honor, delights, fear, sadness, etc.; likewise curiosity, idleness, trifles, loquacity, impatience, pertinacity, inconstancy in proposal, etc.
b) sins against certain virtues annexed to justice, which are not concerned with strict debt nor with a notable good of neighbor, namely, against truth and fidelity, affability, gratitude (by omission), liberality, etc.