y reason of its species, which is determined according to
the good to which that sin is opposed. Now fornication is contrary to
the good of the child to be born. Wherefore it is a graver sin, as to
its species, than those sins which are contrary to external goods,
such as theft and the like; while it is less grievous than those
which are directly against God, and sins that are injurious to the
life of one already born, such as murder.

Reply Obj. 1: The sensual pleasure that aggravates a sin is that
which is in the inclination of the will. But the sensual pleasure
that is in the sensitive appetite, lessens sin, because a sin is the
less grievous according as it is committed under the impulse of a
greater passion. It is in this way that the greatest sensual pleasure
is in fornication. Hence Augustine says (De Agone Christiano [*Serm.
ccxciii; ccl de Temp.; see Appendix to St. Augustine's works]) that of
all a Christian's conflicts, the most difficult combats are those of
chastity; wherein the fight is a daily one, but victory rare: and
Isidore declares (De Summo Bono ii, 39) that "mankind is subjected to
the devil by carnal lust more than by anything else," because, to
wit, the vehemence of this passion is more difficult to overcome.

Reply Obj. 2: The fornicator is said to sin against his own body, not
merely because the pleasure of fornication is consummated in the
flesh, which is also the case in gluttony, but also because he acts
against the good of his own body by an undue resolution and
defilement thereof, and an undue association with another. Nor does
it follow from this that fornication is the most grievous sin,
because in man reason is of greater value than the body, wherefore if
there be a sin more opposed to reason, it will be more grievous.

Reply Obj. 3: The sin of fornication is contrary to the good of the
human race, in so far as it is prejudicial to the individual
begetting of the one man that may be born. Now one who is already an
actual member of the human species attains to the perfection of the
species more than one who is a man potentially, and from this point
of view murder is a more grievous sin than fornication and every kind
of lust, through being more opposed to the good of the human species.
Again, a Divine good is greater than the good of the human race: and
therefore those sins also that are against God are more grievous.
Moreover, fornication is a sin against God, not directly as though