Hume criticizes philosophers for sliding in their writings indefensibly from an “is” to an “ought.”
Suppose then that you tell me: “You ought not to kill.”
To which I reply: “Who are you to tell me how to live? Maybe I feel like killing someone. In fact, I am right now going to go kill Smith, collect his insurance, and then enjoy a nice cake and a cup of coffee to celebrate and laugh at your presumptions.”
To which you reply: “If you do that, I’ll take your life.”
And this makes me pause right away. For, as pointed out already below, I am not facing the choice of killing Smith and getting rich vs. letting Smith live and staying poor. It may well be that my moral scruples will prevent me from committing murder. Or they will not. I’d then weigh the pleasures involved and decide on the most profitable course of action.
No, here I am threatened with the loss of my very life. A dead man cannot enjoy his ill-gotten money or eat cake and drink coffee. Mises makes a similar point in regard to the ideological choice between capitalism and socialism:
A man who chooses between drinking a glass of milk and a glass of a solution of potassium cyanide does not choose between two beverages; he chooses between life and death. A society that chooses between capitalism and socialism does not choose between two social systems; it chooses between social cooperation and the disintegration of society.
Similarly, a man who chooses between committing murder and moral behavior does not choose between two equally reasonable occupations or avocations; he chooses between life as a branch nourishing and being nourished by the vine of society and being cut off from the social body as though a gangrenous foot for the sake of the whole. He chooses between life and the electric chair; ultimately between heaven and hell.
This is not a free choice but a necessity imposed upon us by society and by God. One cannot desire as a result to do otherwise than as he ought and in so doing thwart the anti-Humean.
The argument works for a string of smaller crimes, as well, and not just for first-degree murder. A judge needs to find a deterrent sufficient to cause a criminal to reconsider his lifestyle. As a result, punishment must be ratcheted up each time the criminal is caught and found guilty. The first time Smith steals a car, he gets probation. The second, 1 year in prison. The third, 5 years. At some point the judge will wonder whether Smith is a mad dog who refuses to heed any incentive. In such a case, the judge must protect society from further harm by condemning Smith to death or life imprisonment.
This ties in with my earlier posts about “small” and “large” societies. In the former, such as the Crusoe-Friday society on their deserted island, injustice is directly irrational and makes no sense from the purely self-interested “economic” standpoint of the potential criminal. In the latter, crime becomes unprofitable through threat of punishment by the state.
Regarding interpersonal morality, I still favor (a) cognitivism, (b) duty-based motivation, and (c) internalism.