the Philosopher states (De
Hist. Animal. x, 1). Therefore weakness of the soul is when the soul
is hindered from fulfilling its proper action on account of a
disorder in its parts. Now as the parts of the body are said to be
out of order, when they fail to comply with the order of nature, so
too the parts of the soul are said to be inordinate, when they are
not subject to the order of reason, for the reason is the ruling
power of the soul's parts. Accordingly, when the concupiscible or
irascible power is affected by any passion contrary to the order of
reason, the result being that an impediment arises in the aforesaid
manner to the due action of man, it is said to be a sin of weakness.
Hence the Philosopher (Ethic. vii, 8) compares the incontinent man to
an epileptic, whose limbs move in a manner contrary to his intention.

Reply Obj. 1: Just as in the body the stronger the movement against
the order of nature, the greater the weakness, so likewise, the
stronger the movement of passion against the order of reason, the
greater the weakness of the soul.

Reply Obj. 2: Sin consists chiefly in an act of the will, which is
not hindered by weakness of the body: for he that is weak in body may
have a will ready for action, and yet be hindered by a passion, as
stated above (A. 1). Hence when we speak of sins of weakness, we
refer to weakness of soul rather than of body. And yet even weakness
of soul is called weakness of the flesh, in so far as it is owing to
a condition of the flesh that the passions of the soul arise in us
through the sensitive appetite being a power using a corporeal organ.

Reply Obj. 3: It is in the will's power to give or refuse its consent
to what passion inclines us to do, and it is in this sense that our
appetite is said to be under us; and yet this consent or dissent of
the will is hindered in the way already explained (A. 1).
________________________

FOURTH ARTICLE [I-II, Q. 77, Art. 4]

Whether Self-love Is the Source of Every Sin?

Objection 1: It would seem that self-love is not the source of every
sin. For that which is good and right in itself is not the proper
cause of sin. Now love of self is a good and right thing in itself:
wherefore man is commanded to love his neighbor as himself (Lev.
19:18). Therefore self-love cannot be the proper cause of sin.

Obj. 2: Further, the Apostle says (Rom. 7:8): "Sin taking occasion by
the commandment wrought in me all manner of concupiscence"; on