You can't logically derive what you ought to do from what merely is

David Hume noticed that moral arguments quietly slide from describing how things are to prescribing how they should be, without ever explaining the jump. No amount of factual description, he argued, adds up to an obligation on its own — a gap philosophers still call the is-ought problem, or Hume's Guillotine.

— David Hume, A Treatise of Human Nature — Book III, Part I, Section I, 1739 — public domain
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