Objection 1: It would seem that the aforesaid vices do not arise from lust. For inconstancy arises from envy, as stated above (A. 5, ad 2). But envy is a distinct vice from lust.
Obj. 2: Further, it is written (James 1:8): "A double-minded man is inconstant in all his ways." Now duplicity does not seem to pertain to lust, but rather to deceitfulness, which is a daughter of covetousness, according to Gregory (Moral. xxxi, 45). Therefore the aforesaid vices do not arise from lust.
Obj. 3: Further, the aforesaid vices are connected with some defect of reason. Now spiritual vices are more akin to the reason than carnal vices. Therefore the aforesaid vices arise from spiritual vices rather than from carnal vices.
On the contrary, Gregory declares (Moral. xxxi, 45) that the aforesaid vices arise from lust.
I answer that, As the Philosopher states (Ethic. vi, 5) "pleasure above all corrupts the estimate of prudence," and chiefly sexual pleasure which absorbs the mind, and draws it to sensible delight. Now the perfection of prudence and of every intellectual virtue consists in abstraction from sensible objects. Wherefore, since the aforesaid vices involve a defect of prudence and of the practical reason, as stated above (AA. 2, 5), it follows that they arise chiefly from lust.
Reply Obj. 1: Envy and anger cause inconstancy by drawing away the reason to something else; whereas lust causes inconstancy by destroying the judgment of reason entirely. Hence the Philosopher says (Ethic. vii, 6) that "the man who is incontinent through anger listens to reason, yet not perfectly, whereas he who is incontinent through lust does not listen to it at all."
Reply Obj. 2: Duplicity also is something resulting from lust, just as inconstancy is, if by duplicity we understand fluctuation of the mind from one thing to another. Hence Terence says (Eunuch. act 1, sc. 1) that "love leads to war, and likewise to peace and truce."
Reply Obj. 3: Carnal vices destroy the judgment of reason so much the more as they lead us away from reason.