Is-Ought Fallacy

“Teens are always on their phones, so schools should allow phones in class.”

You commit the is–ought fallacy when you jump from a description of how things are to a rule about how things should be without giving a reason to connect the two. David Hume introduced this problem in 1739 in A Treatise of Human Nature.

It’s not a fallacy if you give reasons for making that step from is to ought. For example: “Teens use phones a lot. When rules ignore common behavior, compliance drops and teaching time is wasted. To protect learning, we should allow short phone breaks.” This is not a fallacy because a goal (protecting learning) and a mechanism (better compliance) link the fact to the recommendation.

This confusion also appears when people treat forecasts as endorsements. I once heard a story about a man whose partner got mad at him when he predicted that a certain politician (whom she didn’t like) would win the election. She mistakenly thought he’d committed the is–ought fallacy: he made a descriptive statement about how he thought things would be, and she took it as a prescriptive statement about how he thought things should be. “He is likely to win” does not mean “He deserves to win,” so his statement wasn’t necessarily one of support for the politician.

A similar error is the ought–is fallacy: assuming that because something should be true, it is true. For example: “People should read full articles before sharing, so misinformation isn’t a big problem.”

Next fallacy (Fallacy Fallacy)

Back to the Logical Fallacy Handbook

You can buy a printable version of this handbook.