A

logical fallacy in which the

conclusion of a

categorical syllogism is

affirmative but one of the

terms is

negative.

Example:

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".