Objection 1. It would seem that a sin committed through passion should not be called a sin of weakness. For a passion is a vehement movement of the sensitive appetite, as stated above (Article 1). Now vehemence of movements is evidence of strength rather than of weakness. Therefore a sin committed through passion, should not be called a sin of weakness.
Objection 2. Further, weakness in man regards that which is most fragile in him. Now this is the flesh; whence it is written (Psalm 77:39): "He remembered that they are flesh." Therefore sins of weakness should be those which result from bodily defects, rather than those which are due to a passion.
Objection 3. Further, man does not seem to be weak in respect of things which are subject to his will. Now it is subject to man's will, whether he do or do not the things to which his passions incline him, according to Genesis 4:7: "Thy appetite shall be under thee [Vulgate: 'The lust thereof shall be under thee.'], and thou shalt have dominion over it." Therefore sin committed through passion is not a sin of weakness.
On the contrary, Cicero (De Quaest. Tusc. iv) calls the passions diseases of the soul. Now weakness is another name for disease. Therefore a sin that arises from passion should be called a sin of weakness.
I answer that, The cause of sin is on the part of the soul, in which, chiefly, sin resides. Now weakness may be applied to the soul by way of likeness to weakness of the body. Accordingly, man's body is said to be weak, when it is disabled or hindered in the execution of its proper action, through some disorder of the body's parts, so that the humors and members of the human body cease to be subject to its governing and motive power. Hence a member is said to be weak, when it cannot do the work of a healthy member, the eye, for instance, when it cannot see clearly, as 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 to Objection 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 to Objection 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 (Article 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 to Objection 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 (1).
Source: Summa Theologica (New Advent)