Objection 1. It would seem that there is no law of the "fomes" of sin. For Isidore says (Etym. v) that the "law is based on reason." But the "fomes" of sin is not based on reason, but deviates from it. Therefore the "fomes" has not the nature of a law.
Objection 2. Further, every law is binding, so that those who do not obey it are called transgressors. But man is not called a transgressor, from not following the instigations of the "fomes"; but rather from his following them. Therefore the "fomes" has not the nature of a law.
Objection 3. Further, the law is ordained to the common good, as stated above (Question 90, Article 2). But the "fomes" inclines us, not to the common, but to our own private good. Therefore the "fomes" has not the nature of sin.
On the contrary, The Apostle says (Romans 7:23): "I see another law in my members, fighting against the law of my mind."
I answer that, As stated above (2; 90, 1, ad 1), the law, as to its essence, resides in him that rules and measures; but, by way of participation, in that which is ruled and measured; so that every inclination or ordination which may be found in things subject to the law, is called a law by participation, as stated above (2; 90, 1, ad 1). Now those who are subject to a law may receive a twofold inclination from the lawgiver. First, in so far as he directly inclines his subjects to something; sometimes indeed different subjects to different acts; in this way we may say that there is a military law and a mercantile law. Secondly, indirectly; thus by the very fact that a lawgiver deprives a subject of some dignity, the latter passes into another order, so as to be under another law, as it were: thus if a soldier be turned out of the army, he becomes a subject of rural or of mercantile legislation.
Accordingly under the Divine Lawgiver various creatures have various natural inclinations, so that what is, as it were, a law for one, is against the law for another: thus I might say that fierceness is, in a way, the law of a dog, but against the law of a sheep or another meek animal. And so the law of man, which, by the Divine ordinance, is allotted to him, according to his proper natural condition, is that he should act in accordance with reason: and this law was so effective in the primitive state, that nothing either beside or against reason could take man unawares. But when man turned his back on God, he fell under the influence of his sensual impulses: in fact this happens to each one individually, the more he deviates from the path of reason, so that, after a fashion, he is likened to the beasts that are led by the impulse of sensuality, according to Psalm 48:21: "Man, when he was in honor, did not understand: he hath been compared to senseless beasts, and made like to them."
So, then, this very inclination of sensuality which is called the "fomes," in other animals has simply the nature of a law (yet only in so far as a law may be said to be in such things), by reason of a direct inclination. But in man, it has not the nature of law in this way, rather is it a deviation from the law of reason. But since, by the just sentence of God, man is destitute of original justice, and his reason bereft of its vigor, this impulse of sensuality, whereby he is led, in so far as it is a penalty following from the Divine law depriving man of his proper dignity, has the nature of a law.
Reply to Objection 1. This argument considers the "fomes" in itself, as an incentive to evil. It is not thus that it has the nature of a law, as stated above, but according as it results from the justice of the Divine law: it is as though we were to say that the law allows a nobleman to be condemned to hard labor for some misdeed.
Reply to Objection 2. This argument considers law in the light of a rule or measure: for it is in this sense that those who deviate from the law become transgressors. But the "fomes" is not a law in this respect, but by a kind of participation, as stated above.
Reply to Objection 3. This argument considers the "fomes" as to its proper inclination, and not as to its origin. And yet if the inclination of sensuality be considered as it is in other animals, thus it is ordained to the common good, namely, to the preservation of nature in the species or in the individual. And this is in man also, in so far as sensuality is subject to reason. But it is called "fomes" in so far as it strays from the order of reason.
Source: Summa Theologica (New Advent)