The fallacy of the undistributed middle (Latin: non distributio medii) is a formal fallacy that is committed when the middle term in a categorical syllogism is not distributed in either the minor premise or the major premise.
The fallacy of the undistributed middle occurs when the term that links the two premises is never distributed.
Therefore, it cannot be used to connect students and my grandfather—both of them could be separate and unconnected divisions of the class of backpack carriers.
Note below how "carries a backpack" is truly undistributed: Specifically, the structure of this example results in affirming the consequent.
Again, note below that "student" is distributed: The fallacy of the undistributed middle is referenced in Edgar Allan Poe's detective story The Purloined Letter: This functionary, however, has been thoroughly mystified; and the remote source of his defeat lies in the supposition that the Minister is a fool because he has acquired renown as a poet.