The is/ought gap

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The is/ought gap is the observation in philosophy that facts are separate from values. Judgements of value, as in moral or aesthetic judgements, cannot be determined true or false in the same way as statements of fact.

The gap is important in metaethics, as some moral systems attempt to deny or circumvent the boundary. Some moral objectivists invoke a supernatural being to universalize their values, whilst moral realists collapse the boundary by attempting to show that at least some values are embedded in facts.

Hume's Law

David Hume described the gap in his A Treatise of Human Nature (1739-40). Hume points out that some philosophers mistakenly transition from “is” statements—what Hume describes as “the ordinary way of reasoning”—to “ought” statements in the course of their arguments. By “is” statements, Hume means descriptive statements that could be judged as true or false. One might claim for example that “God is the creator of the universe.” Such a sentence could be the premise in an argument about God:

  • God is the creator of the universe.
  • The universe is 6,000 years old.
  • Therefore, God created the universe 6,000 years ago.

This is a valid argument that can be contested factually. However, consider this version:

  • God is the creator of the universe.
  • The universe is one’s home.
  • Therefore, one ought to respect God.

Hume points out that a move like this from is to ought is a fallacy. This conclusion is a moral statement directing one’s actions, which doesn’t follow from the premises. “It is necessary,” Hume explains, “that [ought-statements] 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.” [1] He takes this as further evidence of his belief that morals are not discoverable by reason.

Additional Context

Hume’s comments on the is-ought gap appear in his book at the end of the section “Moral Distinctions Not Derived from Reason.” Throughout this section, Hume is concerned with whether it’s true, as some philosophers contend, that morals are universal and accessible like matters of fact to reasoning human beings. The problem for Hume is that reason and morality are inescapably different. For Hume, reason is an operation of the mind that has no influence on actions. It’s a tool for discovering truth or falsehood, for coming to agreements or disagreements about the relations of ideas or matters of fact. It can neither approve of or disprove of actions; it can only compare and describe. Conversely, morals are instructions for producing or preventing actions. Because morals directly influence behavior, Hume confines morality to the world of sentiment and feeling, and concludes that the merits we ascribe to actions arise from our passions, not our faculty for reason.

Hume entertains a number of thought experiments demonstrating some of the absurdities that arise when morality is construed factually. For example, he describes the destruction of an oak tree when one of its seeds has grown in its place and asks whether this event proves that the younger tree is guilty of parricide. It doesn’t matter for Hume that the younger tree doesn’t have a will and couldn’t have intended parricide like a person can. His point is that if one can’t treat trees as capable of immoral acts, then one must admit that morality has a non-factual source.

Statement of the claim The is/ought gap
Level of certainty Proven
Nature Factual
Counterclaim Denial of the is/ought gap
Dependent on

Dependency of