ages have contemplated the “why be nice?” question for centuries. Philosophers have studied the world as it is, to see if they could discover from their observations and their reasoning how humans ought to treat each other. But this endeavor runs into a serious snag. It’s called the Is-Ought Problem, and it comes to us courtesy of David Hume:
In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprized to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, it is necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.
This has come to be called Hume’s Law. To sum it up: You cannot derive an ought (a prescription for behavior) from an is (an observation of nature).
I’ve noticed that, in their efforts to formulate an irreligious system of morality, some atheist philosophers have simply brushed aside Hume’s Law. I’m sympathetic to their efforts, but you can’t just brush aside the Is-Ought Problem.
Let us assume that David Hume is absolutely correct when he says that “vice and virtue are not discoverable merely by reason, or the comparison of ideas.” Is it possible to formulate an irreligious morality that, rather than running from Hume’s Law, fully embraces it?
-  A Treatise of Human Nature, p335, See http://books.google.com/books?id=-Sp8B0ZdyAYC&pg=PA335#v=snippet&q=%22every%20system%20of%20morality%22&f=false ↩