Moral skepticism

And I share the insight of Hume's Sensible Knave. However, human sentiment and inclination ensures that this insight is unlikely to confound the moral sense.

From Enquiry Concerning the Principles of Morals, Section IX:

Treating vice with the greatest candour, and making it all possible concessions, we must acknowledge, that there is not, in any instance, the smallest pretext for giving it the preference above virtue, with a view to self-interest; except, perhaps, in the case of justice, where a man, taking things in a certain light, may often seem to be a loser by his integrity. And though it is allowed, that, without a regard to property, no society could subsist; yet, according to the imperfect way in which human affairs are conducted, a sensible knave, in particular incidents, may think, that an act of iniquity or infidelity will make a considerable addition to his fortune, without causing any considerable breach in the social union and confederacy. That honesty is the best policy, may be a good general rule; but is liable to many exceptions: And he, it may, perhaps, be thought, conducts himself with most wisdom, who observes the general rule, and takes advantage of all the exceptions.

I must confess, that, if a man think, that this reasoning much requires an answer, it will 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 his practice will be answerable to his speculation. But in all ingenuous natures, the antipathy to treachery and roguery is too strong to be counterbalanced by any views of profit or pecuniary advantage. 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.