Undistributed middle

Formal fallacies → Syllogistic / quantification

Undistributed middle is when two things are both in the same big group and you conclude they must be the same as each other. Sharing a category doesn't make them identical. The middle term (the category they share) isn't used in a way that connects the two other terms to each other. It's a very common error: we see "A and B are both X" and wrongly infer "A is B."

Examples

  • All dogs are animals. All cats are animals. So all dogs are cats.

  • All laptops are computers. All desktops are computers. So all laptops are desktops.

  • All salmon are fish. All cod are fish. So all salmon are cod.

  • All nurses work in healthcare. All doctors work in healthcare. So all nurses are doctors.

  • All jazz is music. All classical is music. So all jazz is classical.

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