Objection 1. It would seem that the goodness of the will does not depend on reason. For what comes first does not depend on what follows. But the good belongs to the will before it belongs to reason, as is clear from what has been said above (Question 9, Article 1). Therefore the goodness of the will does not depend on reason.
Objection 2. Further, the Philosopher says (Ethic. vi, 2) that the goodness of the practical intellect is "a truth that is in conformity with right desire." But right desire is a good will. Therefore the goodness of the practical reason depends on the goodness of the will, rather than conversely.
Objection 3. Further, the mover does not depend on that which is moved, but vice versa. But the will moves the reason and the other powers, as stated above (Question 9, Article 1). Therefore the goodness of the will does not depend on reason.
On the contrary, Hilary says (De Trin. x): "It is an unruly will that persists in its desires in opposition to reason." But the goodness of the will consists in not being unruly. Therefore the goodness of the will depends on its being subject to reason.
I answer that, As stated above (1,2), the goodness of the will depends properly on the object. Now the will's object is proposed to it by reason. Because the good understood is the proportionate object of the will; while sensitive or imaginary good is proportionate not to the will but to the sensitive appetite: since the will can tend to the universal good, which reason apprehends; whereas the sensitive appetite tends only to the particular good, apprehended by the sensitive power. Therefore the goodness of the will depends on reason, in the same way as it depends on the object.
Reply to Objection 1. The good considered as such, i.e. as appetible, pertains to the will before pertaining to the reason. But considered as true it pertains to the reason, before, under the aspect of goodness, pertaining to the will: because the will cannot desire a good that is not previously apprehended by reason.
Reply to Objection 2. The Philosopher speaks here of the practical intellect, in so far as it counsels and reasons about the means: for in this respect it is perfected by prudence. Now in regard to the means, the rectitude of the reason depends on its conformity with the desire of a due end: nevertheless the very desire of the due end presupposes on the part of reason a right apprehension of the end.
Reply to Objection 3. The will moves the reason in one way: the reason moves the will in another, viz. on the part of the object, as stated above (Question 9, Article 01).
Source: Summa Theologica (New Advent)