A logical fallacy
in which a categorical syllogism
is used with four terms
. Often, the use of four terms is not obvious
; watch for Equivocation
All men are mortal.
No women are men.
Therfore, no women are mortal.
("men" in the first is in the sense of human beings, while in the second in the sense of male humans)
To prove that four terms are being used, identify them. See Equivocation if necessary.