or to be suffered voluntarily (as when a man voluntarily gives to
another more than he owes him).

Reply Obj. 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 Obj. 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 Obj. 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.
_______________________

FOURTH ARTICLE [II-II, Q. 59, Art. 4]

Whether Whoever Does an Injustice Sins Mortally?

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 ignoran