A logical fallacy in which the conclusion of a categorical syllogism is affirmative but one of the terms is negative.

  Some quadrilaterals are squares.
  No squares have three sides.
  Therefore, some quadrilaterals have three sides.

To prove the fallacy, assume both premises are true and find an example that fits but contradicts the conclusion. In the example above, the premises are true, but the conclusion contradicts the definition of a quadrilateral: "a polygon with four sides".

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