Objection 1. It would seem that the will cannot be moved by the sensitive appetite. For "to move and to act is more excellent than to be passive," as Augustine says (Gen. ad lit. xii, 16). But the sensitive appetite is less excellent than the will which is the intellectual appetite; just as sense is less excellent than intellect. Therefore the sensitive appetite does not move the will.
Objection 2. Further, no particular power can produce a universal effect. But the sensitive appetite is a particular power, because it follows the particular apprehension of sense. Therefore it cannot cause the movement of the will, which movement is universal, as following the universal apprehension of the intellect.
Objection 3. Further, as is proved in Phys. viii, 5, the mover is not moved by that which it moves, in such a way that there be reciprocal motion. But the will moves the sensitive appetite, inasmuch as the sensitive appetite obeys the reason. Therefore the sensitive appetite does not move the will.
On the contrary, It is written (James 1:14): "Every man is tempted by his own concupiscence, being drawn away and allured." But man would not be drawn away by his concupiscence, unless his will were moved by the sensitive appetite, wherein concupiscence resides. Therefore the sensitive appetite moves the will.
I answer that, As stated above (Article 1), that which is apprehended as good and fitting, moves the will by way of object. Now, that a thing appear to be good and fitting, happens from two causes: namely, from the condition, either of the thing proposed, or of the one to whom it is proposed. For fitness is spoken of by way of relation; hence it depends on both extremes. And hence it is that taste, according as it is variously disposed, takes to a thing in various ways, as being fitting or unfitting. Wherefore as the Philosopher says (Ethic. iii, 5): "According as a man is, such does the end seem to him."
Now it is evident that according to a passion of the sensitive appetite man is changed to a certain disposition. Wherefore according as man is affected by a passion, something seems to him fitting, which does not seem so when he is not so affected: thus that seems good to a man when angered, which does not seem good when he is calm. And in this way, the sensitive appetite moves the will, on the part of the object.
Reply to Objection 1. Nothing hinders that which is better simply and in itself, from being less excellent in a certain respect. Accordingly the will is simply more excellent than the sensitive appetite: but in respect of the man in whom a passion is predominant, in so far as he is subject to that passion, the sensitive appetite is more excellent.
Reply to Objection 2. Men's acts and choices are in reference to singulars. Wherefore from the very fact that the sensitive appetite is a particular power, it has great influence in disposing man so that something seems to him such or otherwise, in particular cases.
Reply to Objection 3. As the Philosopher says (Polit. i, 2), the reason, in which resides the will, moves, by its command, the irascible and concupiscible powers, not, indeed, "by a despotic sovereignty," as a slave is moved by his master, but by a "royal and politic sovereignty," as free men are ruled by their governor, and can nevertheless act counter to his commands. Hence both irascible and concupiscible can move counter to the will: and accordingly nothing hinders the will from being moved by them at times.
Source: Summa Theologica (New Advent)