Objection 1. It would seem that the degree of goodness in the will depends on the degree of good in the intention. Because on Matthew 12:35, "A good man out of the good treasure of his heart bringeth forth that which is good," a gloss says: "A man does as much good as he intends." But the intention gives goodness not only to the external action, but also to the act of the will, as stated above (Article 7). Therefore the goodness of a man's will is according to the goodness of his intention.
Objection 2. Further, if you add to the cause, you add to the effect. But the goodness of the intention is the cause of the good will. Therefore a man's will is good, according as his intention is good.
Objection 3. Further, in evil actions, a man sins in proportion to his intention: for if a man were to throw a stone with a murderous intention, he would be guilty of murder. Therefore, for the same reason, in good actions, the will is good in proportion to the good intended.
On the contrary, The intention can be good, while the will is evil. Therefore, for the same reason, the intention can be better, and the will less good.
I answer that, In regard to both the act, and the intention of the end, we may consider a twofold quantity: one, on the part of the object, by reason of a man willing or doing a good that is greater; the other, taken from the intensity of the act, according as a man wills or acts intensely; and this is more on the part of the agent.
If then we speak of these respective quantities from the point of view of the object, it is evident that the quantity in the act does not depend on the quantity in the intention. With regard to the external act this may happen in two ways. First, through the object that is ordained to the intended end not being proportionate to that end; for instance, if a man were to give ten pounds, he could not realize his intention, if he intended to buy a thing worth a hundred pounds. Secondly, on account of the obstacles that may supervene in regard to the exterior action, which obstacles we are unable to remove: for instance, a man intends to go to Rome, and encounters obstacles, which prevent him from going. On the other hand, with regard to the interior act of the will, this happens in only one way: because the interior acts of the will are in our power, whereas the external actions are not. But the will can will an object that is not proportionate to the intended end: and thus the will that tends to that object considered absolutely, is not so good as the intention. Yet because the intention also belongs, in a way, to the act of the will, inasmuch, to wit, as it is the reason thereof; it comes to pass that the quantity of goodness in the intention redounds upon the act of the will; that is to say, in so far as the will wills some great good for an end, although that by which it wills to gain so great a good, is not proportionate to that good.
But if we consider the quantity in the intention and in the act, according to their respective intensity, then the intensity of the intention redounds upon the interior act and the exterior act of the will: since the intention stands in relation to them as a kind of form, as is clear from what has been said above (12, 4; 18, 6). And yet considered materially, while the intention is intense, the interior or exterior act may be not so intense, materially speaking: for instance, when a man does not will with as much intensity to take medicine as he wills to regain health. Nevertheless the very fact of intending health intensely, redounds, as a formal principle, upon the intense volition of medicine.
We must observe, however, that the intensity of the interior or exterior act, may be referred to the intention as its object: as when a man intends to will intensely, or to do something intensely. And yet it does not follow that he wills or acts intensely; because the quantity of goodness in the interior or exterior act does not depend on the quantity of the good intended, as is shown above. And hence it is that a man does not merit as much as he intends to merit: because the quantity of merit is measured by the intensity of the act, as we shall show later on (20, 04; 114, 4).
Reply to Objection 1. This gloss speaks of good as in the estimation of God, Who considers principally the intention of the end. Wherefore another gloss says on the same passage that "the treasure of the heart is the intention, according to which God judges our works." For the goodness of the intention, as stated above, redounds, so to speak, upon the goodness of the will, which makes even the external act to be meritorious in God's sight.
Reply to Objection 2. The goodness of the intention is not the whole cause of a good will. Hence the argument does not prove.
Reply to Objection 3. The mere malice of the intention suffices to make the will evil: and therefore too, the will is as evil as the intention is evil. But the same reasoning does not apply to goodness, as stated above (ad 2).
Source: Summa Theologica (New Advent)