The Middle Term Is Everything

The middle term is where most logical errors hide. It has to connect cleanly to both other terms, so if it means one thing in the major premise and something else in the minor, the argument falls apart.

Take: "All dogs are animals. All cats are animals. Therefore, all dogs are cats." The middle term animals fails because neither premise talks about all animals, only some — so it never pins dogs and cats together. This is the fallacy of the undistributed middle.

Think of the middle term as a hinge on a door. If the hinge is weak or misaligned, the door will not work. In the same way, a vague middle term swings the argument open to nonsense.