Is there such a thing as a ‘law of the stimulant [to sin]’?

It seems that there is no such thing as a ‘law of the stimulant to sin (fomes [peccati]):

Objection 1:  In Etymologia 5 Isidore says, “The law is founded upon reason.”  But the stimulant to sin does not consist in reason, but rather deviates from reason.  Therefore, the stimulant to sin does not have the character of law.

Objection 2:  Every law is obligatory in the sense that anyone who does not keep it is called a transgressor.  But the stimulant to sin does not render anyone a transgressor by virtue of his not following it; to the contrary, he is rendered a transgressor if he does follow it.  Therefore, the stimulus to sin does not have the character of law.

Objection 3:  As was established above (q. 90, a. 2), law is ordered toward the common good.  But the stimulant to sin inclines one not toward the common good, but instead toward his own private good.  Therefore, the stimulant to sin does not have the character of law.

But contrary to this:  In Romans 7:23 the Apostle says, “I see another law in my members, fighting against the law of my mind.”

I respond:  As was explained above (a. 2), law exists in an essential way in that which rules and measures, whereas it exists by way of participation in that which is measured and ruled—so that, as is clear from what was said above, every inclination or ordering that is found in things that are subject to the law is called ‘law’ by way of participation.
Now there are two ways in which an inclination stemming from the lawmaker can be found in things that are subject to the law:  (a) in one way, insofar as such an inclination directly inclines the things subject to it toward something, and sometimes diverse subjects to diverse acts, in the way that military law (lex militum) can be said to be different from business law (lex mercatorum); (b) in another way, indirectly, viz., insofar as the fact that the lawmaker takes away some office (dignitas) from one who is subject to him results in the latter’s passing into another order and, as it were, into another law.  For instance, if a soldier is discharged from the army, then he will pass into rural law or business law.
So, then, under God the lawmaker different creatures have different natural inclinations, with the result that what is in some way law for one is contrary to what is law for another.  For instance, being fierce is in some sense the law for a dog, whereas it is contrary to the law for a sheep or some other gentle animal.
Thus, the law for man, which is given by divine ordination according to man’s proper condition, is that he should act in accord with to reason.  This law was, to be sure, so strong in man’s initial state that nothing either beyond reason or contrary to reason could sneak up on man.  But once man turned away from God, he fell into being carried away by the impetus of sensuality; and this happens in a particular way to each man the more he recedes from reason, so that he becomes in a certain sense like the beasts, which are carried away by the impetus of sensuality—this according to Psalm 48:21 (“Man, when he existed in honor, did not understand:  he has been put on the same footing as senseless beasts and been made similar to them”).
So, then, this inclination toward sensuality, which is called the ‘stimulant’ (fomes), has the character of law absolutely speaking in the case of the other animals—yet in the manner in which it can be called ‘law’ in such animals, viz., as a direct inclination.  In men, by contrast, the stimulant does not have the character of law in this way, but is rather a deviation from the law of reason.  Yet insofar as man was stripped of original justice and of vigorous reason through God’s justice, this impetus to sensuality which leads him on has the character of law in the sense that it is a punishment and follows from God’s law, now that man has been stripped of his proper dignity.

Reply to objection 1:  This argument proceeds from the stimulant considered by itself, insofar as it inclines one to evil.  For, as has been explained, it does not in this sense have the character of law.
Instead, it has the character of law insofar as it follows from the justice of God’s law—in the way that one might call it a law that a nobleman should, because of some sin, be subjected to the work of a servant.

Reply to objection 2:  This objection proceeds on the assumption that the stimulant is a law in the sense of a rule and measure; for those who deviate from the law in this sense are rendered transgressors.  However, the stimulant is not a law in this sense, but is instead a law by participation of a certain sort, in the way explained above.

Reply to objection 3:  This argument proceeds from the stimulant’s proper inclination and not from its origin.  Yet if the inclination toward sensuality is considered as it exists in other animals, then it is indeed ordered to the common good, i.e., to the conservation of nature in the species and in the individual.  And this is also true in the case of man, to the extent that sensuality is subject to reason.  However, the name ‘stimulant’ is used for it insofar as it departs from the order of reason.