The Is-Ought Gap


  • Prior (1960) "The Auonomy of Ethics" Australasian Journal of Philosophy.
  • Geach (1977) "Against the Logic of 'Ought'" Philosophy
  • Geach (1982) "Moral Autonomy still Refuted" Philosophy.
  • Karmo (1988) "Some Valid (but no sound) Arguments Trivially Span the 'Is'-'Ought' Gap" in Mind Vol. XCVII, pp.252-257.

The gap

No value / normative statement is entailed by any set of factual statements.

  • It is the case that p, It is a good thing that p.
  • X is part of social convention, X is morally right / X should continue.
  • Most people believe that X is wrong, X is wrong
  • Everyone is doing X, X is morally acceptable.
  • X is unnatural, X is wrong


Hume (1739) A Treatise of Human Nature. Book III, part I, section I.

@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.@

Bridging the is-ought gap

Examples from Geach and Prior. Let "P" be a normative statement.

  • P entailed by "Everything that A says is true.", "A says that P."
  • If "P or Q" is normative, then a factual statement Q can entail "P or Q". If "P or Q" is not normative, then ~Q, P or Q entails P.
  • "All As are Bs" entails "As ought to do what Bs ought to do." which is arguably normative.