No pile of facts, Hume warned, adds up to a single moral duty
In one short paragraph of his 1739 Treatise, David Hume noticed moralists sliding from describing how things are to declaring how they ought to be, without explaining the jump. That quiet observation, nicknamed Hume's guillotine, became one of the central puzzles of ethics: can any purely factual premise ever yield an obligation?
Hume complained that writers would reason normally for a while, proving God exists or describing human affairs, then suddenly every sentence contained an ought. He thought the shift, though barely noticeable, mattered enormously and demanded justification. Modern versions of Hume's law say that non-moral premises alone cannot logically deliver a moral conclusion. Trick derivations exist: from the sun being yellow one can infer that either the sun is yellow or murder is wrong. But such moves give no real guidance, defenders argue.
Combine the gap with Hume's fork, the idea that all knowledge comes either from logic and definitions or from observation, and moral knowledge seems to vanish, since oughts fit neither category. Moral sceptics and non-cognitivists embrace that result. G. E. Moore's open-question argument pushes a related line against anyone who equates goodness with a natural property.
Ethical naturalists reply that goals bridge the gap. If an agent wants B, and C is the way to B, then the agent ought to do C, a claim that can be checked against the world. Alasdair MacIntyre adds that Western moral language grew up around the idea of a human purpose, so calling a person good is like calling blunt scissors bad. Critics answer that this just relocates the problem into the choice of goals. A poisoner who laments that he ought to have used more poison shows that a goal-based ought need not be moral at all.
Other escape routes are still contested. John Searle argued that making a promise creates an obligation by definition, then developed the notion of institutional facts, like paper counting as money, to meet his critics. Discourse ethicists say that arguing at all commits participants to certain oughts. Elizabeth Anscombe went the opposite way, suggesting the moral sense of ought should be abandoned altogether if people could manage it psychologically. The debate also overlaps with the broader fact–value distinction, which reaches into aesthetics.
Source: Is–ought problem
