Is-ought problem

The is–ought problem, as articulated by Scottish philosopher and historian David Hume (1711–76), states that many writers make claims about what ought to be on the basis of statements about what is. Hume found that there seems to be a significant difference between positive statements (about what is) and prescriptive or normative statements (about what ought to be), and that it is not obvious how one can coherently move from descriptive statements to prescriptive ones. The is–ought problem is also known as Hume's law, or Hume's guillotine.

A similar view is defended by G. E. Moore's open-question argument, intended to refute any identification of moral properties with natural properties. This so-called naturalistic fallacy stands in contrast to the views of ethical naturalists.

Hume discusses the problem in book III, part I, section I of his book, A Treatise of Human Nature (1739):

In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprised 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 should be observed and explained; 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. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.

...
Wikipedia