What is the is-ought problem?

The is-ought problem is the question of whether moral statements about what ought to be can be inferred from objective statements about what is. It was first articulated by the 18th-century Scottish philosopher David Hume, who saw a significant difference between descriptive statements and prescriptive ones, and who argued that it is not obvious how one can coherently move from the first kind to the second. Hume is usually assigned the position that such a derivation is impossible. [1]

Philosophers often describe the issue as a fact-value distinction. Descriptive claims state how the world is, such as 'the weather today is sunny'. Evaluative claims express a judgment of value, and prescriptive ones say what should be done, such as 'I should go outside to get some sunshine'. That second claim rests on a descriptive one, but it also adds a value, here that sunshine is good for mental health, and that value is what yields the recommended action. The same source notes that people sometimes mistake an evaluative statement for a plain claim about how things are. [2]

The point comes from a short passage in Hume's A Treatise of Human Nature (1739–1740). Hume observed that moral writers often reason in the ordinary way about God or human affairs and then suddenly begin using 'ought' and 'ought not' instead of 'is' and 'is not'. He said this new relation needs to be observed and explained, along with a reason for how it can be a deduction from claims that are entirely different from it. His own explanation, as summarized by OpenStax, was that morality has to do with sentiments rather than facts. [2]

In modern usage, 'Hume's law' or 'Hume's guillotine' usually names the thesis that someone with only non-moral factual premises cannot logically infer a moral conclusion. A broader version holds that evaluative statements, including aesthetic ones, cannot be inferred from non-evaluative ones. [1]

The Stanford Encyclopedia of Philosophy reads Hume as holding that no set of plain factual claims entails an evaluative claim. On this reading, one can reach an evaluative conclusion only if at least one evaluative premise is also present. Inferring that something bad is happening from the fact that someone is in pain, for example, presupposes that pain is bad. A philosopher's illustration makes the same point: 'torturing animals for fun causes much suffering' does not validly yield 'we ought not to torture animals for fun' until an ought premise is added. [3] [4]

The Stanford Encyclopedia says there is substantial debate both about what Hume meant and about whether he was right. The question prompted by his brief paragraph has become one of the central questions of ethical theory. The results provided do not set out the rival positions in detail, so this article does not describe them. [3] [1]

A related idea comes from G. E. Moore, who in Principia Ethica (1903) described the naturalistic fallacy and offered the open-question argument. Moore held that goodness is not a natural property, so claims that something is good can always be questioned, even once its natural properties are known. Ethical naturalists, however, do not regard the naturalistic fallacy as a fallacy. [2] [1]