Hume's law (sometimes called "Hume's Guillotine"), addressing what is referred to as the "is-ought" problem, states that you can't derive an "ought" from an "is". The law is also a rebuttal of the naturalistic fallacy, or inferring how the world ought to be from the way it is or was in the past.
 Hume's is-ought distinction
After noticing people attempted this derivation anyway without sufficient logical steps to connect an "is" and their "ought", it was noted by Scottish philosopher David Hume who described it thus:
|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.|
The is-ought problem has become prominent in matters of ethics and meta-ethics. Simply put, it deals with an apparent logic gap between statements of what "ought" to be, following statements regarding what "is". The first often following the second without any kind of explanation regarding why they are logical or correct.
Furthermore, it argues that just because someone has knowledge of how the world is (descriptive statements), this doesn't automatically prove that he knows how the world ought to be (prescriptive statements) and it is in fact impossible to derive the second based solely on the information of the first.
A number of attempts have been made to solve the is-ought problem, usually involving some type of invocation of natural law. One mode of attack has been to resurrect a form of Aristotelian teleology. Another approach was John Searle's definition of social obligations as "institutional facts," thus allowing is to be derived from ought. None have gained widespread support as a solution to the problem.
A simple solution is through the addition of a goal to the formulation. The problem can therefore be bypassed with a simple if: "If you want to do X, you should do Y which has been shown to lead to X." Sam Harris espouses a view similar to this when dealing with the issue of morality, but one that would generally need two additions rather than one: "If morality has to deal with not causing the suffering of conscious creatures, and if you want to live a moral life, you should take actions that don't cause the suffering of conscious creatures." This hinges on accepting this as a definition of morality, where as some would claim that there is no reason to accept that morality should or could be defined in this (utilitarian) manner, however these same people would probably be using a very abstract (or possibly none at all) definition of morality where morality can not be defined as long as the universe lacks overall "objective" purpose.
The is-ought distinction is sometimes misconstrued to mean that facts are totally disconnected from ethical statements, or that there is no relationship at all between is and ought. As can be seen, Hume does not argue this position, but states that a factual statement (or "is") needs to be combined with an ethical principle or assumption before an ethical statement (or "ought") can be derived.
 See also
- Appeal to nature: The fallacy which states "X is natural, therefore X is good," sometimes misidentified as the naturalistic fallacy.
- Social Darwinism
- ↑ Hume's Guillotine, Julia Galef, Rationally Speaking
- ↑ A Treatise on Human Nature, Volume II, Book III
- ↑ Stilley, Shalina, "Natural Law Theory and the "Is"--"Ought" Problem: A Critique of Four Solutions" (2010). Dissertations (2009 -). Paper 57.
- ↑ David Sloan Wilson, Eric Dietrich, and Ann B. Clark. On the Inappropriate Use of the Naturalistic Fallacy in Evolutionary Psychology. Biology and Philosophy, 18: 669-682, 2003.