directly, i.e. by its own form, one habit
cannot incline its subject to contraries. But there is no reason why
it should not do so, indirectly and accidentally, i.e. by the removal
of an obstacle: thus, when the harmony of a mixed body is destroyed,
the elements have contrary local tendencies. In like manner, when the
harmony of original justice is destroyed, the various powers of the
soul have various opposite tendencies.

Reply Obj. 3: Original sin infects the different parts of the
soul, in so far as they are the parts of one whole; even as original
justice held all the soul's parts together in one. Consequently there
is but one original sin: just as there is but one fever in one man,
although the various parts of the body are affected.
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THIRD ARTICLE [I-II, Q. 82, Art. 3]

Whether Original Sin Is Concupiscence?

Objection 1: It would seem that original sin is not concupiscence.
For every sin is contrary to nature, according to Damascene (De Fide
Orth. ii, 4, 30). But concupiscence is in accordance with nature,
since it is the proper act of the concupiscible faculty which is a
natural power. Therefore concupiscence is not original sin.

Obj. 2: Further, through original sin "the passions of sins" are in
us, according to the Apostle (Rom. 7:5). Now there are several other
passions besides concupiscence, as stated above (Q. 23, A. 4).
Therefore original sin is not concupiscence any more than another
passion.

Obj. 3: Further, by original sin, all the parts of the soul are
disordered, as stated above (A. 2, Obj. 3). But the intellect is the
highest of the soul's parts, as the Philosopher states (Ethic. x, 7).
Therefore original sin is ignorance rather than concupiscence.

_On the contrary,_ Augustine says (Retract. i, 15): "Concupiscence is
the guilt of original sin."

_I answer that,_ Everything takes its species from its form: and it
has been stated (A. 2) that the species of original sin is taken from
its cause. Consequently the formal element of original sin must be
considered in respect of the cause of original sin. But contraries
have contrary causes. Therefore the cause of original sin must be
considered with respect to the cause of original justice, which is
opposed to it. Now the whole order of original justice consists in
man's will being subject to God: which subjection, first and chiefly,
was in the will, whose function it is to move all the other parts to
the end, as sta