Source: Robert Louden, a virtue ethicist, considers the idea of the good will and the desires which are in tension with it in Kant’s idea of the good person.

As Robert Paul Wolff remarks, it is’noteworthy that Kant, the philosopher most completely identified with the doctrine of stern duty should begin, not with a statement about what we ought to do, but rather with a judgment of what is unqualifiedly good’.8 And what is unqualifiedly good, according to Kant, is not an end-state such as pleasure or the performance of certain acts in conformity to rules, but a state of character which becomes the basis for all of one’s actions. To answer the question:

‘Is my will good?’ (a question which can never be answered with certain knowledge,due to the opacity of our intentions), we must look beyond acts and decisions and inquire into how we have lived. A man cannot be ‘morally good in some ways and at the same time morally evil in others’. Similarly, he cannot, on Kant’s view, exhibit a good will one moment and an evil one the next. Steadfastness of character must be demonstrated.

So Kant’s opening claim concerning the unqualified goodness of the good will means that what is fundamentally important in his ethics is not acts but moral agents. But what is the relationship between ‘good will’ and virtue? Kant defines virtue  as ‘fortitude in relation to the forces opposing a moral attitude of will in us’.The Kantian virtuous agent is thus one who, because of his ‘fortitude’, is able to resist urges and inclinations opposed to the moral law. Kantian fortitude is strength or force of will, not in the sense of being able to accomplish the goals one sets out to achieve, but rather in the sense of mastery over one’s inclinations and constancy of purpose.

A good will is a will which steadily acts from the motive of respect for the moral law. But human beings, because they are natural beings, always possess inclinations which maylead them to act against reason.
Their wills are thus in a perpetual state of tension. Some wills are better than others, but only a holy will (who has no wants that could run counter to reason, and who can thus do no evil) possesses an absolutelygood will. This is why Kant holds that ‘human morality in its highest stages can still be nothing more than virtue’.

Virtue is only an approximation of the good will, because of the basic conflict or tension in human wills. Kant’s virtuous agent is a human approximation of a good will who through strength of mind continually acts out of respect for the moral law while still feeling the presence of natural inclinations which could tempt him to act from other motives.

Want more?  Try this discussion of the motive of duty: http://carneades.pomona.edu/1998-2006/2005-Hume/Notes/MotiveDuty.shtml