The fallacy of the undistributed middle is a logical fallacy, and more specifically a formal fallacy, that is committed when the middle term in a categorical syllogism is not distributed in the major premise. It is thus a syllogistic fallacy.
In classical syllogisms, all statements consist of two terms and are in the form of "A" (all), "E" (none), "I" (some), or "O" (some not). The first term is distributed in A statements; the second is distributed in O statements; both are distributed in E statements; and none are distributed in I statements.
The fallacy of the undistributed middle occurs when the term that links the two premises is never distributed.
In this example, distribution is marked in boldface:……
Note: This is the last of the Formal Fallacies. Next week we will begin to explore the Informal Fallacies, which I think will be of more use to everyone. They will also be far easier to understand....at least most of the time.