"In every system of morality, 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. For as this ought, or ought not, expresses some new relation or affirmation, it's 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.
David Hume, book III, part I, section I of his book, A Treatise of Human Nature (1739):
David Hume, book III, part I, section I of his book, A Treatise of Human Nature (1739):