> In meta-ethics, the is-ought problem was raised by David Hume, who
> noted that many writers make claims about what ought to be on the
> basis of statements about what is. However, there seems to be a
> significant 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 A
> 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 all 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,that 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
> inferences, 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"? In other words, given
> our knowledge of the way the world is, how can we know the way the
> world ought to be? 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.
> This complete severing of "is" from "ought" has been given the graphic
> designation of "Hume's Guillotine".
>
> 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'.
>
> http://en.wikipedia.org/wiki/Is-ought_problem