Twice in my recent philosophical wanderings a I have bumped into David Hume's sensible knave.
Hume's sensible knave is a person who realizes the benefits of living in a community of just and moral individuals - people who do not lie or take the property of others without consent, who keep promises and repay debts, who refrain from violence except in defense of the innocent, who prefer true beliefs to harmful fictions, and the like.
At the same time the sensible knave recognizes that, if he lies just this once, or takes some available property without consent, the whole institution of justice and morality will not collapse. He can obtain the benefits of this immoral act and still preserve the benefits of living in a just and moral society.
Is he ignoring some reason to be moral? If so, what is it?
Hume says that perhaps he is not missing anything - that his reasoning may be functioning perfectly. What he lacks are the correct sentiments - the correct interests.
I must confess that, if a man think that this reasoning much requires an answer, it would be a little difficult to find any which will to him appear satisfactory and convincing. If his heart rebel not against such pernicious maxims, if he feel no reluctance to the thoughts of villainy or baseness, he has indeed lost a considerable motive to virtue; and we may expect that this practice will be answerable to his speculation.Desirism recognizes this as the distinction between the desires an agent has and the desires that people generally have reason to cause agent's to have. Insofar as knave's "heart not rebel against such pernicious doctrine," the knave has not acquired the desires people generally have reason to promote. Those desires would prevent an agent from wanting to engage in the immoral actions.
There is, for the knave, a gap between the desires the agent has the desires the agent should have. The size of that gap determines the quality of a person's moral character.
Perhaps to try to prevent tempting people towards this option, Hume claims that his knave is at risk of slipping up and making a mistake that will cost him more than he has gained. Furthermore, according to Hume:
Inward peace of mind, consciousness of integrity, a satisfactory review of our own conduct; these are circumstances, very requisite to happiness, and will be cherished and cultivated by every honest man, who feels the importance of them.However, he might not get caught, and he also need not suffer any pains of self-condemnation. These are contingent facts the knave may avoid. If this is his situation, then, why be moral?
Note that knavery will not tempt the moral person. The moral person - just like the knave - does what he wants and avoids what he dislikes. However, he includes among his wants some desires like the desire to help others in need. Among his dislikes, he includes such things as an aversion to lying and taking the property of others without their consent. These likes and dislikes direct his behavior in the same way that liking chocolate and disliking liver and onions would direct his behavior.
Christine Korsgaard referred to Hume's knave in her book "Normativity" to explain her theory of reflective endorsement to justify moral claims.
For Korsgaard, the knave, recognizing the virtues of honesty and generosity, will recognize himself as lacking these virtues. This means that he recognizes himself as lacking qualities which he has reason to praise and promote in others. Similarly, he recognizes himself as having qualities he would condemn in others. Such a person must also see himself as somebody who is undeserving of praise and deserving of condemnation. The principles of morality are justified, according to Korsgaard, because they survive not only as the sources of evaluation but as the objects of evaluation.
On the other hand, Peter Railton argues that the knave need to suffer these types of problems ("Moral Realism", The Philosophical Review, Vol. 95, No. 2 (Apr., 1986), pp 163-207).
Yet Knave himself could say, perhaps because he accepts Hume's analysis of justice, "Yes, my attitude is unjust." And by Hume's own account of the relation of reason and passion, Knave could add "But what is that to me?" without failing to grasp the content of his previous assertion.The fact that an individual does not have the desires that people generally have reason to support does not imply that he lacks the capacity to judge whether an action is one that would be done by somebody who has those interests. This is no more a problem then the fact that a person has no fear of flying makes it impossible for her to predict the behavior of somebody who has such a fear.
We therefore must distinguish the business of saying what an individual values from the business of saying what it is for him to make measurements against the criteria of a species of evaluation that he recognizes to be genuine.When there is a gap between what a person desires and what they "should desire", you cannot use reason to convince that person to behave morally. In the short term, you need to appeal to his desires directly. You need to offer to reward him (to fulfill an existing desire) for behaving morally or punish him (thwart an existing desire) if he should fail. In the long term, one can use rewards (such as praise) and punishments (such as condemnation) to actually change those desires - to give the agent a desire to keep promises, repay debts, and help those in need. But this requires rewards and punishment - not reason.