Objection 1. It would seem that devotion is not a special act. That which qualifies other acts is seemingly not a special act. Now devotion seems to qualify other acts, for it is written (2 Chronicles 29:31): "All the multitude offered victims, and praises, and holocausts with a devout mind." Therefore devotion is not a special act.
Objection 2. Further, no special kind of act is common to various genera of acts. But devotion is common to various genera of acts, namely, corporal and spiritual acts: for a person is said to meditate devoutly and to genuflect devoutly. Therefore devotion is not a special act.
Objection 3. Further, every special act belongs either to an appetitive or to a cognitive virtue or power. But devotion belongs to neither, as may be seen by going through the various species of acts of either faculty, as enumerated above (I, 78, seqq.; I-II, 23, 4). Therefore devotion is not a special act.
On the contrary, Merits are acquired by acts as stated above (I-II, 21, A034). But devotion has a special reason for merit. Therefore devotion is a special act.
I answer that, Devotion is derived from "devote" [The Latin 'devovere' means 'to vow']; wherefore those persons are said to be "devout" who, in a way, devote themselves to God, so as to subject themselves wholly to Him. Hence in olden times among the heathens a devotee was one who vowed to his idols to suffer death for the safety of his army, as Livy relates of the two Decii (Decad. I, viii, 9; x, 28). Hence devotion is apparently nothing else but the will to give oneself readily to things concerning the service of God. Wherefore it is written (Exodus 35:20-21) that "the multitude of the children of Israel... offered first-fruits to the Lord with a most ready and devout mind." Now it is evident that the will to do readily what concerns the service of God is a special kind of act. Therefore devotion is a special act of the will.
Reply to Objection 1. The mover prescribes the mode of the movement of the thing moved. Now the will moves the other powers of the soul to their acts, and the will, in so far as it regards the end, moves both itself and whatever is directed to the end, as stated above (I-II, 09, 3). Wherefore, since devotion is an act of the will whereby a man offers himself for the service of God Who is the last end, it follows that devotion prescribes the mode to human acts, whether they be acts of the will itself about things directed to the end, or acts of the other powers that are moved by the will.
Reply to Objection 2. Devotion is to be found in various genera of acts, not as a species of those genera, but as the motion of the mover is found virtually in the movements of the things moved.
Reply to Objection 3. Devotion is an act of the appetitive part of the soul, and is a movement of the will, as stated above.
Source: Summa Theologica (New Advent)