Undistributed Middle
Concluding two things are the same just because they share a common property.
- formal
The fallacy of the undistributed middle is an invalid syllogism: it links two categories because each overlaps a shared middle term, even though that middle term never covers all of either one. “All A are C; all B are C; therefore all A are B” simply doesn't follow — sharing a property doesn't make two groups identical.
Examples
- “All dogs are mammals. All cats are mammals. Therefore dogs are cats.”
- “Successful founders take big risks. My uncle takes big risks. So my uncle must be a successful founder.”
Why it works & how to counter
Persuasive because the shared property feels like a real link. Counter it by noticing that many different groups can share the same trait — overlapping with a common category doesn't merge the groups.