is-ought problem

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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 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, 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"".

Wikipedia contributors, ""Is-ought problem,"" Wikipedia, The Free Encyclopedia, (accessed January 31, 2007).