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Undistributed Middle

The middle term in the premises of a standard form categorical syllogism never refers to all of the members of the category it describes.

Examples
  1. "All Russians were revolutionists, and all anarchists were revolutionist, therefore, all anarchists were Russians." The middle term is 'revolutionist'. While both Russians and anarchists share the common property of being revolutionist, they may be separate groups of revolutionists, and so we cannot conclude that anarchists are otherwise the same as Russians in any way. Example from Copi and Cohen, 208.
  2. "All trespassers are shot, and someone was shot, therefore, someone was a trespasser." The middle term is 'shot'. While 'someone' and 'trespassers' may share the property of being shot, it doesn't follow that the someone in question was a trespasser; he may have been the victim of a mugging.
Critique
Show how each of the two categories identified in the conclusion could be separate groups even though they share a common property.

About Stephen Downes

Many kudos are owed to Stephen Downes, the logician who originally authored this list of fallacies and made it freely available on the Web. His basic structure remains. A mirror of Downes’ original site can be found here.