Summa Theologiae Moralis, vol. I: De Principiis

S. Th. I-II, q. 88, a. 2; De Malo, q. 2, a. 8; q. 7, a. 3.

543. Mortal and venial sin are some by their genus and of themselves, some accidentally. — Mortal sin is divided into mortal of itself or by genus, and mortal accidentally: the former, of itself, by its own notion, from its proper object or matter, from the specific notion of itself can be mortal; the latter is not mortal except from the part of the agent or act a circumstance acceding to the species. Mortal of itself is divided into mortal from the whole genus which by its species is always mortal, and mortal from its genus which in its species admits lightness of matter.

Also venial sin is divided into venial of itself and venial accidentally: the latter by its notion and from the object is mortal, and is not venial except by a circumstance acceding from the part of the agent, such as imperfect advertence; the former is venial by its notion and from the object, and is divided into: venial by its genus which of itself, from its proper object and from the specific notion of itself cannot be mortal, and venial from lightness of matter which although from its specific notion and proper matter it can be mortal, because of the small quantity of matter about which it is concerned is venial.

Leaving aside therefore the sins which are grave and light accidentally, the celebrated division remains into mortal from the whole genus, mortal from its genus, and venial from its genus¹. This distinction is of itself, according to the proper matter and species of sins, whence also the word: genus, here signifies species.

544. Mortal sins from the whole genus are called those which have no light matter, because in any part of the matter about which they are concerned, the full notion of inordination is preserved². 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 against some divine perfection, i.e. by which imperfection or defect of perfection is directly imputed to God, whether they are directly opposed to the theological virtues as infidelity, heresy, despair, and hatred of God; or are against religion: explicit cult of a creature, i.e. idolatry or divination or magic at least with explicit pact with the devil, temptation of God strictly so called, blasphemy and perjury; or there is any explicit and formal contempt of God³;

b) sins which, if in them there were lightness of matter, the genus of man would greatly entice to sin with its great harm, such as lust directly and of itself voluntarily admitted outside marriage; and the direct breaking of the sacramental seal which would soon turn men away from the sacrament of Penance;

c) sins whose matter is a thing grave in the indivisible, or whose malice in any matter equally repugns a grave end intended by law, such as simony of natural right, homicide, duel, the delight of lust on its own account sought outside marriage which is always directly opposed to the very order and primary end of nature, violation of the fast before communion, omission of paschal communion or of the mixture of water with wine in Mass, simulation of a sacrament, etc.

545. Mortal sins from its genus are those which can be mortal or venial according as their proper matter about which they are concerned is grave or light, contains a grave or light inordination; whence they are mortal from the quantity of matter, and the genus contains mortal and venial. Such are those which are concerned with a finite and divisible good:

a) sins against the virtue of religion or precepts of the first tablet which are not directly against God, but only against worship, such as undue cult of God, temptation of God broadly so called, sacrilege, violation of a vow or promissory oath, unjust 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 goods of person, honor, riches; or inflict grave detriment on the sinner in intrinsic goods such as mutilation of body, drunkenness, detriment of health; likewise sins which are directly against the charity of neighbor such as hatred and envy; in one word sins against the precepts of the second tablet, excepting homicide and lust voluntary of themselves;

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 its genus are those which, the same matter remaining, are never mortal, because whatever be the quantity of matter about which they are concerned, they do not contain grave inordination. They are venial from the quality of matter, and the whole genus is venial. Whence they are not mortal unless the malice of another species accedes. Such are generally:

a) sins against one's own good committed, especially against the virtues annexed to fortitude or temperance (e.g. against magnanimity, magnificence, patience, perseverance, modesty, studiousness, etc.), which, grave harm being excluded, consist in immoderate appetite or excessive use of an honest or indifferent thing, e.g. food, drink, sleep, laughter, use of marriage, goods, money, cult, clothing, praise, honor, delights, fear, sadness, etc.; likewise curiosity, idleness, trifles, loquacity, impatience, obstinacy, inconstancy in resolve, etc.

b) sins against certain annexed virtues of justice, which are not concerned with strict debt nor with notable good of neighbor, namely against truth and fidelity, affability, gratitude (by omission), liberality, etc.