What is the definition of sin and the primary basis for the distinction of sins?

Sin is a voluntary, inordinate act and, as such, is opposed to virtue since an act of virtue is ordered to reason.  The traditional definition is “any word, deed or desire contrary to the Eternal Law.“  Now three things oppose virtue.  As stated, sin opposes virtue as an inordinate act contrary to acts of virtue.  Secondly, vice is a disposition to sin and, as “man’s good is to be in accord with reason”,¹ vice disposes a man in a manner not befitting his nature.  Thirdly, malice is opposed to virtue as virtue is a kind of goodness and malice leads the powers of the soul to war against a man’s intent to do good.² It is important to remember that sin is only such in regards to being voluntary.  Reduced voluntariness reduces culpability just as increased voluntariness increases culpability.

Sin is a sickness of the soul for by sinning man acts against “the order of reason which…expresses human nature.”³ The greatest sickness is that which disposes a man to that which is contrary to his nature and it follows that a love of sin is a sign of a gravely deformed will.  However, even though this is the case, a disposition to evil is not worse than an evil act.  This is because a disposition to evil (vice) is a habit which lies between power and act, and, as such, man is not forced to act evilly since the vicious habit “causes act by way of efficient causuality”⁴ not as the act causes habit which is by way of final casuality.⁵ Simply put, it is “more blameworthy to do evil, than to be able to do evil”⁶ because, for example, a man with a vicious habit can still act well.

Sin, as a negation of freedom, does not require a deed for it to be a sin.  Sins without deed are known as sins of omission where one does not do as one ought.  St. Thomas teaches that omission is sinful when “the cause or occasion [of sin] be subject to the will.”⁷  For example, if a man does not attend Sunday Mass due to an illness, this is outside of his power and is not sinful.  If, however, he wills to do something else instead, this is sinful (a mortal sin in fact) or too, if he were to get drunk the night before and miss Mass, he would be responsible for omitting to go to Mass.

Mortal sin, of which I just mentioned, can be distinguished from venial sin by degree not necessarily object.  Sins do differ according to their objects as St. Thomas says, “sins are properly distinguished in species by their objects”,⁸ but venial and mortal sins can share an object.  For example, theft of a small amount is a venial sin while theft of a great amount is a moral sin, but both are turned to the same object.  “The difference between venial and mortal sin is consequent to the diversity of that which constitutes the notion of sin” and “mortal and venial sins are infinitely apart as regards to what they turn away from, not as regards what they turn to…nothing hinders the same species from including mortal and venial sins.”⁹

The Church has always held that sins can be divided into thought, word, and deed.  These sins are not separate species, but “these three differ in respect to the various degrees of sin.”¹⁰ Sin of thought is the foundation, followed by sin of word, and the complete species is consummated in the sin of deed. For example, “Anger begins with a desire for vengeance, which disturbs someone in thought and if severe breaks into words of abuse and finally deeds like striking someone or murder.”¹¹

Contrary to what many profess today, circumstances cannot change the species of a sin, e.g. an objectively evil act remains such regardless of circumstances.  As St. Thomas clarifies, “A circumstance never transfers an act from one species to another, save when there is another motive.”¹²  Ignorance of a circumstance diminishes a sin and likewise circumstances can actually increase the gravity of a sin for example where a man knowingly fornicates with a married woman which is adultery.  “Sins which flow from ignorance or emotional weakness weaken the judgment and so compromise deliberation…this diminishes the voluntary nature of the deed,”¹³ but, even so, do not change that the object of an objectively evil act; ignorance and weakness can affect subjective guilt however.

“The gravity of the sin is determined…by its object”¹⁴ and sins can be against God, self, or neighbor and are more grave according to that order.  Sins directly against God are the most grave, but every sin ultimately is an offense against God hence the need to ask God’s forgiveness for all our sins.  Gravity of a sin is increased due to who the sin is against.  For example, murdering a president (which would be a sin against neighbor) greatly impacts the common good and society, and is therefore worse than simple murder.

Foreseen and intended harm directly increases gravity of the sin; foreseen but not intended harm indirectly increases gravity of the sin and quantity of the harm aggravates the sin; harm neither foreseen or intended does not aggravate directly if it is connected to the sin accidentally.  “If on the other hand, the harm follow directly from the sinful act, although it be neither foreseen nor intended, it aggravates the sin directly, because whatever is directly consequent to a sin, belongs, in a manner, to the very species of that sin; for instance, if a man is a notorious fornicator, the result is that many are scandalized; and although such was not his intention, nor was it perhaps foreseen by him, yet it aggravates his sin directly.”¹⁵

Finally, the more excellent the person, the graver the sin if the sin is deliberate.  This is because the more excellent person can better resist sin and “shows ingratitude to God for his gifts.”¹⁶

¹ Summa I-II Q72, A2
² Fr. Mullady. Lesson 8. Holy Apostles College and Seminary.
³ Fr. Mullady. Lesson 8. HACS.
⁴ Summa I-II Q71, A3
⁵ Summa I-II Q71, A3
⁶ Summa I-II Q71, A3
⁷ Summa I-II Q71, A5
⁸ Summa I-II Q72, A1
⁹ Summa I-II Q72, A5
¹⁰ Summa I-II Q72, A7
¹¹ Fr. Mullady. Lesson 9. HACS.
¹² Summa I-II Q72, A9
¹³ Fr. Mullady. Lesson 9. HACS.
¹⁴ Fr. Mullady. Lesson 9. HACS.
¹⁵ Summa I-II Q73, A8
¹⁶ Fr. Mullady. Lesson 9. HACS.