In ethics, the is-ought problem was raised by David Hume (Scottish philosopher and historian, 1711-1776), who noted that many writers talk about what ought to be on the basis of statements about what is. But there seems to be a big difference between descriptive statements (what is) and prescriptive statements (what ought to be).
Hume calls for writers to be on their guard against changing the subject like that, 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.
The problem is a result of applying "Hume's fork", the now-generally-accepted view, established by Hume, that a statement is meaningful only if it is true for either logical or empirical reasons. Since the "ought-statements" are not derived from "is-statements", and the matters of empirical fact upon which they might rest are not specifiable, then ought-statements have a dubious validity. This opens up the possibility of a radical moral scepticism, and this possibility has not yet been satisfactorily refuted by moral philosophers since the time of Hume. In effect, moral statements stand without any solid basis of truth at the present time.
A similar thesis was argued by G. E. Moore's 'open question argument', intended to refute any identification of moral properties with natural properties -- the so-called 'naturalistic fallacy'.
Any ethical theorist who now wishes to give morality an objective grounding in objective features of the world is fighting an uphill battle.
Nevertheless, John Searle designed an argument purportedly demonstrating that it is possible to derive an "ought" from an "is". 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". See one counter-argument to this.
