Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We must now consider injustice, under which head there are four points of inquiry:
(1) Whether injustice is a special vice?
(2) Whether it is proper to the unjust man to do unjust deeds?
(3) Whether one can suffer injustice willingly?
(4) Whether injustice is a mortal sin according to its genus?
Objection 1: It would seem that injustice is not a special vice. For it is written (1 Jn. 3:4): "All sin is iniquity [*Vulg.: 'Whosoever committeth sin, committeth also iniquity; and sin is iniquity']." Now iniquity would seem to be the same as injustice, because justice is a kind of equality, so that injustice is apparently the same as inequality or iniquity. Therefore injustice is not a special sin.
Objection 2: Further, no special sin is contrary to all the virtues. But injustice is contrary to all the virtues: for as regards adultery it is opposed to chastity, as regards murder it is opposed to meekness, and in like manner as regards the other sins. Therefore injustice is not a special sin.
Objection 3: Further, injustice is opposed to justice which is in the will. But every sin is in the will, as Augustine declares (De Duabus Anim. x). Therefore injustice is not a special sin.
On the contrary, Injustice is contrary to justice. But justice is a special virtue. Therefore injustice is a special vice.
I answer that, Injustice is twofold. First there is illegal injustice which is opposed to legal justice: and this is essentially a special vice, in so far as it regards a special object, namely the common good which it contemns; and yet it is a general vice, as regards the intention, since contempt of the common good may lead to all kinds of sin. Thus too all vices, as being repugnant to the common good, have the character of injustice, as though they arose from injustice, in accord with what has been said above about justice (Q[58], AA[5],6). Secondly we speak of injustice in reference to an inequality between one person and another, when one man wishes to have more goods, riches for example, or honors, and less evils, such as toil and losses, and thus injustice has a special matter and is a particular vice opposed to particular justice.
Reply to Objection 1: Even as legal justice is referred to human common good, so Divine justice is referred to the Divine good, to which all sin is repugnant, and in this sense all sin is said to be iniquity.
Reply to Objection 2: Even particular justice is indirectly opposed to all the virtues; in so far, to wit, as even external acts pertain both to justice and to the other moral virtues, although in different ways as stated above (Q[58], A[9], ad 2).
Reply to Objection 3: The will, like the reason, extends to all moral matters, i.e. passions and those external operations that relate to another person. On the other hand justice perfects the will solely in the point of its extending to operations that relate to another: and the same applies to injustice.
Objection 1: It would seem that a man is called unjust through doing an unjust thing. For habits are specified by their objects, as stated above (FS, Q[54], A[2]). Now the proper object of justice is the just, and the proper object of injustice is the unjust. Therefore a man should be called just through doing a just thing, and unjust through doing an unjust thing.
Objection 2: Further, the Philosopher declares (Ethic. v, 9) that they hold a false opinion who maintain that it is in a man's power to do suddenly an unjust thing, and that a just man is no less capable of doing what is unjust than an unjust man. But this opinion would not be false unless it were proper to the unjust man to do what is unjust. Therefore a man is to be deemed unjust from the fact that he does an unjust thing.
Objection 3: Further, every virtue bears the same relation to its proper act, and the same applies to the contrary vices. But whoever does what is intemperate, is said to be intemperate. Therefore whoever does an unjust thing, is said to be unjust.
On the contrary, The Philosopher says (Ethic. v, 6) that "a man may do an unjust thing without being unjust."
I answer that, Even as the object of justice is something equal in external things, so too the object of injustice is something unequal, through more or less being assigned to some person than is due to him. To this object the habit of injustice is compared by means of its proper act which is called an injustice. Accordingly it may happen in two ways that a man who does an unjust thing, is not unjust: first, on account of a lack of correspondence between the operation and its proper object. For the operation takes its species and name from its direct and not from its indirect object: and in things directed to an end the direct is that which is intended, and the indirect is what is beside the intention. Hence if a man do that which is unjust, without intending to do an unjust thing, for instance if he do it through ignorance, being unaware that it is unjust, properly speaking he does an unjust thing, not directly, but only indirectly, and, as it were, doing materially that which is unjust: hence such an operation is not called an injustice. Secondly, this may happen on account of a lack of proportion between the operation and the habit. For an injustice may sometimes arise from a passion, for instance, anger or desire, and sometimes from choice, for instance when the injustice itself is the direct object of one's complacency. In the latter case properly speaking it arises from a habit, because whenever a man has a habit, whatever befits that habit is, of itself, pleasant to him. Accordingly, to do what is unjust intentionally and by choice is proper to the unjust man, in which sense the unjust man is one who has the habit of injustice: but a man may do what is unjust, unintentionally or through passion, without having the habit of injustice.
Reply to Objection 1: A habit is specified by its object in its direct and formal acceptation, not in its material and indirect acceptation.
Reply to Objection 2: It is not easy for any man to do an unjust thing from choice, as though it were pleasing for its own sake and not for the sake of something else: this is proper to one who has the habit, as the Philosopher declares (Ethic. v, 9).
Reply to Objection 3: The object of temperance is not something established externally, as is the object of justice: the object of temperance, i.e. the temperate thing, depends entirely on proportion to the man himself. Consequently what is accidental and unintentional cannot be said to be temperate either materially or formally. In like manner neither can it be called intemperate: and in this respect there is dissimilarity between justice and the other moral virtues; but as regards the proportion between operation and habit, there is similarity in all respects.
Objection 1: It would seem that one can suffer injustice willingly. For injustice is inequality, as stated above (A[2]). Now a man by injuring himself, departs from equality, even as by injuring another. Therefore a man can do an injustice to himself, even as to another. But whoever does himself an injustice, does so involuntarily. Therefore a man can voluntarily suffer injustice especially if it be inflicted by himself.
Objection 2: Further, no man is punished by the civil law, except for having committed some injustice. Now suicides were formerly punished according to the law of the state by being deprived of an honorable burial, as the Philosopher declares (Ethic. v, 11). Therefore a man can do himself an injustice, and consequently it may happen that a man suffers injustice voluntarily.
Objection 3: Further, no man does an injustice save to one who suffers that injustice. But it may happen that a man does an injustice to one who wishes it, for instance if he sell him a thing for more than it is worth. Therefore a man may happen to suffer an injustice voluntarily.
On the contrary, To suffer an injustice and to do an injustice are contraries. Now no man does an injustice against his will. Therefore on the other hand no man suffers an injustice except against his will.
I answer that, Action by its very nature proceeds from an agent, whereas passion as such is from another: wherefore the same thing in the same respect cannot be both agent and patient, as stated in Phys. iii, 1; viii, 5. Now the proper principle of action in man is the will, wherefore man does properly and essentially what he does voluntarily, and on the other hand a man suffers properly what he suffers against his will, since in so far as he is willing, he is a principle in himself, and so, considered thus, he is active rather than passive. Accordingly we must conclude that properly and strictly speaking no man can do an injustice except voluntarily, nor suffer an injustice save involuntarily; but that accidentally and materially so to speak, it is possible for that which is unjust in itself either to be done involuntarily (as when a man does anything unintentionally), or to be suffered voluntarily (as when a man voluntarily gives to another more than he owes him).
Reply to Objection 1: When one man gives voluntarily to another that which he does not owe him, he causes neither injustice nor inequality. For a man's ownership depends on his will, so there is no disproportion if he forfeit something of his own free-will, either by his own or by another's action.
Reply to Objection 2: An individual person may be considered in two ways. First, with regard to himself; and thus, if he inflict an injury on himself, it may come under the head of some other kind of sin, intemperance for instance or imprudence, but not injustice; because injustice no less than justice, is always referred to another person. Secondly, this or that man may be considered as belonging to the State as part thereof, or as belonging to God, as His creature and image; and thus a man who kills himself, does an injury not indeed to himself, but to the State and to God. Wherefore he is punished in accordance with both Divine and human law, even as the Apostle declares in respect of the fornicator (1 Cor. 3:17): "If any man violate the temple of God, him shall God destroy."
Reply to Objection 3: Suffering is the effect of external action. Now in the point of doing and suffering injustice, the material element is that which is done externally, considered in itself, as stated above (A[2]), and the formal and essential element is on the part of the will of agent and patient, as stated above (A[2]). Accordingly we must reply that injustice suffered by one man and injustice done by another man always accompany one another, in the material sense. But if we speak in the formal sense a man can do an injustice with the intention of doing an injustice, and yet the other man does not suffer an injustice, because he suffers voluntarily; and on the other hand a man can suffer an injustice if he suffer an injustice against his will, while the man who does the injury unknowingly, does an injustice, not formally but only materially.
Objection 1: It would seem that not everyone who does an injustice sins mortally. For venial sin is opposed to mortal sin. Now it is sometimes a venial sin to do an injury: for the Philosopher says (Ethic. v, 8) in reference to those who act unjustly: "Whatever they do not merely in ignorance but through ignorance is a venial matter." Therefore not everyone that does an injustice sins mortally.
Objection 2: Further, he who does an injustice in a small matter, departs but slightly from the mean. Now this seems to be insignificant and should be accounted among the least of evils, as the Philosopher declares (Ethic. ii, 9). Therefore not everyone that does an injustice sins mortally.
Objection 3: Further, charity is the "mother of all the virtues" [*Peter Lombard, Sent. iii, D. 23], and it is through being contrary thereto that a sin is called mortal. But not all the sins contrary to the other virtues are mortal. Therefore neither is it always a mortal sin to do an injustice.
On the contrary, Whatever is contrary to the law of God is a mortal sin. Now whoever does an injustice does that which is contrary to the law of God, since it amounts either to theft, or to adultery, or to murder, or to something of the kind, as will be shown further on (Q[64], seqq.). Therefore whoever does an injustice sins mortally.
I answer that, As stated above (FS, Q[12], A[5]), when we were treating of the distinction of sins, a mortal sin is one that is contrary to charity which gives life to the soul. Now every injury inflicted on another person is of itself contrary to charity, which moves us to will the good of another. And so since injustice always consists in an injury inflicted on another person, it is evident that to do an injustice is a mortal sin according to its genus.
Reply to Objection 1: This saying of the Philosopher is to be understood as referring to ignorance of fact, which he calls "ignorance of particular circumstances" [*Ethic. iii, 1], and which deserves pardon, and not to ignorance of the law which does not excuse: and he who does an injustice through ignorance, does no injustice except accidentally, as stated above (A[2])
Reply to Objection 2: He who does an injustice in small matters falls short of the perfection on an unjust deed, in so far as what he does may be deemed not altogether contrary to the will of the person who suffers therefrom: for instance, if a man take an apple or some such thing from another man, in which case it is probable that the latter is not hurt or displeased.
Reply to Objection 3: The sins which are contrary to the other virtues are not always hurtful to another person, but imply a disorder affecting human passions; hence there is no comparison.