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