he contrary,_ The species of lust are distinguished according to
the various conditions of women with whom a man has unlawful
intercourse. Now incest implies a special condition on the part of
the woman, because it is unlawful intercourse with a woman related by
consanguinity or affinity as stated (Obj. 2). Therefore incest is a
determinate species of lust.

_I answer that,_ As stated above (AA. 1, 6) wherever we find
something incompatible with the right use of venereal actions, there
must needs be a determinate species of lust. Now sexual intercourse
with women related by consanguinity or affinity is unbecoming to
venereal union on three counts. First, because man naturally owes a
certain respect to his parents and therefore to his other blood
relations, who are descended in near degree from the same parents: so
much so indeed that among the ancients, as Valerius Maximus relates
[*Dict. Fact. Memor. ii, 1], it was not deemed right for a son to
bathe with his father, lest they should see one another naked. Now
from what has been said (Q. 142, A. 4; Q. 151, A. 4), it is evident
that in venereal acts there is a certain shamefulness inconsistent
with respect, wherefore men are ashamed of them. Wherefore it is
unseemly that such persons should be united in venereal intercourse.
This reason seems to be indicated (Lev. 18:7) where we read: "She is
thy mother, thou shalt not uncover her nakedness," and the same is
expressed further on with regard to others.

The second reason is because blood relations must needs live in close
touch with one another. Wherefore if they were not debarred from
venereal union, opportunities of venereal intercourse would be very
frequent and thus men's minds would be enervated by lust. Hence in
the Old Law [*Lev. 18] the prohibition was apparently directed
specially to those persons who must needs live together.

The third reason is, because this would hinder a man from having many
friends: since through a man taking a stranger to wife, all his
wife's relations are united to him by a special kind of friendship,
as though they were of the same blood as himself. Wherefore Augustine
says (De Civ. Dei xv, 16): "The demands of charity are most perfectly
satisfied by men uniting together in the bonds that the various ties
of friendship require, so that they may live together in a useful and
becoming amity; nor should one man have many relationships in one,
but each should have one."

Aristotle adds