Objection 1. It would seem that the sin of morose delectation is not in the reason. For delectation denotes a movement of the appetitive power, as stated above (Question 31, Article 1). But the appetitive power is distinct from the reason, which is an apprehensive power. Therefore morose delectation is not in the reason.
Objection 2. Further, the object shows to which power an act belongs, since it is through the act that the power is directed to its object. Now a morose delectation is sometimes about sensible goods, and not about the goods of the reason. Therefore the sin of morose delectation is not in the reason.
Objection 3. Further, a thing is said to be morose [From the Latin 'mora'--delay] through taking a length of time. But length of time is no reason why an act should belong to a particular power. Therefore morose delectation does not belong to the reason.
On the contrary, Augustine says (De Trin. xii, 12) that "if the consent to a sensual delectation goes no further than the mere thought of the pleasure, I deem this to be like as though the woman alone had partaken of the forbidden fruit." Now "the woman" denotes the lower reason, as he himself explains (De Trin. xii, 12). Therefore the sin of morose delectation is in the reason.
I answer that, As stated (5), sin may be in the reason, not only in respect of reason's proper act, but sometimes in respect of its directing human actions. Now it is evident that reason directs not only external acts, but also internal passions. Consequently when the reason fails in directing the internal passions, sin is said to be in the reason, as also when it fails in directing external actions. Now it fails, in two ways, in directing internal passions: first, when it commands unlawful passions; for instance, when a man deliberately provokes himself to a movement of anger, or of lust: secondly, when it fails to check the unlawful movement of a passion; for instance, when a man, having deliberately considered that a rising movement of passion is inordinate, continues, notwithstanding, to dwell [immoratur] upon it, and fails to drive it away. And in this sense the sin of morose delectation is said to be in the reason.
Reply to Objection 1. Delectation is indeed in the appetitive power as its proximate principle; but it is in the reason as its first mover, in accordance with what has been stated above (Article 1), viz. that actions which do not pass into external matter are subjected in their principles.
Reply to Objection 2. Reason has its proper elicited act about its proper object; but it exercises the direction of all the objects of those lower powers that can be directed by the reason: and accordingly delectation about sensible objects comes also under the direction of reason.
Reply to Objection 3. Delectation is said to be morose not from a delay of time, but because the reason in deliberating dwells [immoratur] thereon, and fails to drive it away, "deliberately holding and turning over what should have been cast aside as soon as it touched the mind," as Augustine says (De Trin. xii, 12).
Source: Summa Theologica (New Advent)