Is it true that you can’t derive an ought from an is?

A common myth about the relationship between facts and values may be hampering our discussions of ethics

[David Hume, from econlib.org.]

Here is one of the most momentous short paragraphs ever written in the history of philosophy:

“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 surprised 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’s 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.”

It was written by David Hume, and it appears in book III, part I, section I of his A Treatise of Human Nature, published in 1739. It is often interpreted as saying that one cannot derive an “ought” from an “is,” or—to put it differently—that there is an unbridgeable gap between values and facts.

If this interpretation of Hume were right, it would follow that moral statements can only be of two kinds: either they refer to a different category of “facts,” somehow entirely separate from facts like “Saturn has rings,” or there simply are no such things as moral facts at all.