Is-ought problem - Definition and Overview

In meta-ethics, the is-ought problem was raised by David Hume (Scottish philosopher and historian, 1711-1776), who noted that many writers make claims about what ought to be on the basis of statements about what is. But there seems to be a big difference between descriptive statements (about what is) and prescriptive statements (about what ought to be).

Hume discusses the problem in book III, part I, section I of his Treatise of Human Nature:

In every system of morality, which I have hitherto met with, I have always remark’d, that the author proceeds for some time in the ordinary ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surpriz’d 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, ’tis necessary that it shou’d be observ’d and explain’d; 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.

Hume then calls for writers to be on their guard against such reasoning, if they cannot give an explanation of how the ought-statements are supposed to follow from the is-statements. But how exactly can you derive an "ought" from an "is"? That question, prompted by Hume's small paragraph, has become one of the central questions of ethical theory, and Hume is usually assigned the position that such a derivation is impossible.

A similar (though distinct) view is defended by G. E. Moore's 'open question argument', intended to refute any identification of moral properties with natural properties — the so-called 'naturalistic fallacy'. And, like the naturalistic fallacy (which is often misunderstood to involve the arguably fallacious inference from 'this is (un)natural' to 'this is (im)moral'), the is-ought problem has been misunderstood as related to a less deep, but more common issue. Namely, many people think the is-ought problem is related to arguably true claim that, just because something is the case, that doesn't mean that it ought to be the case. On this misunderstanding, Hume was arguing against those complacent moralists who hold that the world is just fine as it stands, and needs no improvement. This is, of course, not Hume's point — he meant to challenge the transition from any set of descriptive claims to any prescriptive claim. This is a very general and very deep challenge to any descriptive account of moral thought, a challenge that is in keeping with Hume's anti-rationalist bent as a moral theorist.

Many people find Hume's thesis compelling, and see no hope for grounding moral statements in purely descriptive ones. A handful of arguments have been proposed which claim to show that an "ought" can actually be derived from an "is". John Searle devised one such argument. Basically, it tries to show that the act of making a promise places one under an obligation by definition, and that such an obligation amounts to an "ought". This view has not gained general acceptance.

The apparent gap between "is" statements and "ought" statements, when combined with Hume's fork — the idea that all items of knowledge either are based on logic and definitions or are based on observation — renders "ought" statements of dubious validity. Since "ought" statements don't seem to be known in either of the two ways mentioned, it would seem that there can be no moral knowledge. Two responses to this are moral skepticism and non-cognitivism.