Firmitas

Syllogistic fallacy: Undistributed middle

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Syllogistic Fallacy: Undistributed Middle

This is a fallacy involving three sets: A, B and C. It is the assumption that because all A are members of C, and all B are members of C, that all A are B.

  • All A are members of C.
  • All B are members of C.
  • Therefore all A are B. ]]

Examples

Consider:

  • A. All cherry tomatoes are red things.
  • B. All cherries are red things.
  • C. Therefore all cherry tomatoes are cherries.

Again:

  • A. Fred is an atheist who lives in California.
  • B. Ted is also an atheist living in California.
  • C. Therefore Fed and Ted are actually the same person.

Weaknessess

There is usually no reason to believe that two members of a group are identical or related, nor two subgroups of a group.