n of
uncleanness or effeminacy would seem to be most contrary to nature,
since it would seem especially in accord with nature that agent and
patient should be distinct from one another. Hence it would follow
that uncleanness is the gravest of unnatural vices. But this is not
true. Therefore unnatural vices are not the most grievous among sins
of lust.

_On the contrary,_ Augustine says (De adult. conjug. [*The quotation
is from Cap. Adulterii xxxii, qu. 7. Cf. Augustine, De Bono
Conjugali, viii.]) that "of all these," namely the sins belonging to
lust, "that which is against nature is the worst."

_I answer that,_ In every genus, worst of all is the corruption of
the principle on which the rest depend. Now the principles of reason
are those things that are according to nature, because reason
presupposes things as determined by nature, before disposing of other
things according as it is fitting. This may be observed both in
speculative and in practical matters. Wherefore just as in
speculative matters the most grievous and shameful error is that
which is about things the knowledge of which is naturally bestowed on
man, so in matters of action it is most grave and shameful to act
against things as determined by nature. Therefore, since by the
unnatural vices man transgresses that which has been determined by
nature with regard to the use of venereal actions, it follows that in
this matter this sin is gravest of all. After it comes incest, which,
as stated above (A. 9), is contrary to the natural respect which we
owe persons related to us.

With regard to the other species of lust they imply a transgression
merely of that which is determined by right reason, on the
presupposition, however, of natural principles. Now it is more
against reason to make use of the venereal act not only with
prejudice to the future offspring, but also so as to injure another
person besides. Wherefore simple fornication, which is committed
without injustice to another person, is the least grave among the
species of lust. Then, it is a greater injustice to have intercourse
with a woman who is subject to another's authority as regards the act
of generation, than as regards merely her guardianship. Wherefore
adultery is more grievous than seduction. And both of these are
aggravated by the use of violence. Hence rape of a virgin is graver
than seduction, and rape of a wife than adultery. And all these are
aggravated by coming under the head of