Is-Ought Gap: From Facts to Values


Imagine you are watching a meat-eater and a vegan debate whether we should eat animals. The meat-eater says that eating meat can’t be bad because we evolved to eat meat and get vital nutrients from it. The vegan says that eating meat isn’t actually natural for our bodies. Our intestines are longer than most omnivores, and our canines aren’t as defined as dogs and bears. Therefore, we should stop eating meat and do what is natural for our bodies.


In this conversation, both the meat-eater and the vegan are appealing to facts about nature to reach conclusions about the correct moral action regarding eating animals. However, this is a common fallacy in reasoning known as the is-ought gap, when someone tries to justify why we should do something based on facts about the world.

Definition of the Is-Ought Gap

The is-ought gap is a fallacy that attempts to make conclusions about the way things should be based on the evidence about the way things are. However, there is no theoretical connection between facts about the world and ethical facts. Appealing to nature in moral and political arguments cannot bridge the is-ought gap.

How It Works

Scottish philosopher David Hume first identified the is-ought gap in 1739. He noticed that moral philosophers would often talk about the way things are and then suddenly switch and begin to talk about the way things ought to be. However, there is no reason to bridge these two concepts. The way things are cannot be used as evidence for the way things ought to be. The is-ought gap applies to appeals from biology, evolution, history, and even pleasure. This is why it is a fallacy to say things like, “Everyone else on the highway is driving over the speed limit, so it’s alright for me to as well” or “We can’t change gun laws because gun rights are written in the Constitution. That’s the way things have always been.”

English philosopher G.E. Moore posited a variation to the is-ought gap in 1903 and called it the naturalistic fallacy. He argued that it is wrong to say that something is good because of the natural properties that it possesses. Moral properties like good, bad, right, and wrong are irreducible to natural properties like pleasure and pain and should not be equated with them. Pleasure does not define goodness, and morality cannot be reduced to natural properties.

Applying It

The naturalistic fallacy is among the most widely used fallacies in advertisements and debates concerning current moral and political issues. When someone says sentences like, “Marijuana is illegal, you shouldn’t smoke it” or “Mass incarceration seems to be an issue in America, but we’ve always had prisons. We can’t abolish them,” they are attempting to cross the is-ought gap, and sometimes it’s hard to spot. As Hume notes, it’s a psychological tendency for us to jump from using “is” to using “ought,” even though natural facts do not imply moral facts. Be on the lookout for appeals to nature the next time you are in a conversation with someone, and remember that the way things are is not always the way things should be.

However, how can we ever know how things should be? If morality is not defined by any natural properties, including pleasure or harm, the idea of goodness is unanalyzable; that is, it cannot be broken down into smaller parts. It seems reasonable to reject the notion that eating meat is good just because we evolved to eat it. Yet, it may not seem as reasonable to reject the idea that eating meat is bad because it causes pain to animals or that bullying someone is bad because it makes them cry. Some natural properties should not bridge the gap between is and ought, but it certainly seems that other natural properties can be used to determine morality. To solve this issue, we should reflect on what the good means to us and where we acquire knowledge of it. Is there a way to talk about how we should act and how the world should be without appealing to empirical evidence?

Think Further

  1. Can we ever derive an “ought” from an “is”?
  2. What is the difference between facts and values?
  3. If moral principles are not based on natural properties like pleasure, how should we think of them?


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Learn More

  1. Cohon, Rachel. “Hume’s Moral Philosophy.” The Stanford Encyclopedia of Philosophy, Fall 2018.
  2. The Ethics Centre. “Ethics Explainer: Naturalistic Fallacy.” The Ethics Centre, 15 March 2016.
  3. Pigden, Charles. “Hume on ‘Is’ and ‘Ought’.” Philosophy Now, vol. 83, 2011.
  4. Ridge, Michael. “Moral Non-Naturalism.” The Stanford Encyclopedia of Philosophy. Fall 2019.
  5. Sinclair, Neil. The Naturalistic Fallacy. Cambridge University Press, 2019.