Thefallacy of the undistributed middle (Latin:non distributio medii) is aformal fallacy that is committed when themiddle term in acategorical syllogism is notdistributed in either theminor premise or themajor premise. It is thus asyllogistic 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:
B is the common term between the two premises (the middle term) but is never distributed, so this syllogism is invalid. B would be distributed by introducing a premise which states either All B is Z, or No B is Z.
Also, a related rule of logic is that anything distributed in the conclusion must be distributed in at least one premise.
The middle term—Z—is distributed, but Y is distributed in the conclusion and not in any premise, so this syllogism is invalid.
The fallacy of the undistributed middle takes the following form:
This may be graphically represented as follows:
where the premises are in the green box and the conclusion is indicated above them.
B is the middle term (because it appears in both premises), and it is not distributed in the major premise, "all Z is B".
It may or may not be the case that "all Z is B," but this is irrelevant to the conclusion. What is relevant to the conclusion is whether it is true that "all B is Z," which is ignored in the argument. The fallacy is similar toaffirming the consequent anddenying the antecedent. However, the fallacy may be resolved if the terms are exchanged in either the conclusion or in the firstco-premise. Indeed, from the perspective offirst-order logic, all cases of the fallacy of the undistributed middle are, in fact, examples of affirming the consequent or denying the antecedent, depending on the structure of the fallacious argument.
For example:
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The middle term is the one that appears in bothpremises—in this case, it is the class of backpack carriers. It is undistributed because neither of its uses applies to all backpack carriers. 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 inaffirming the consequent.
However, if the latter two statements were switched, the syllogism would be valid:
In this case, the middle term is the class of students, and the first use clearly refers to 'all students'. It is therefore distributed across the whole of its class, and so can be used to connect the other two terms (backpack carriers, and my grandfather). Again, note below that "student" is distributed:
The fallacy of the undistributed middle is referenced inEdgar Allan Poe's detective storyThe 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. All fools are poets; this the Prefectfeels, and he is merely guilty of anon distributio medii in thence inferring that all poets are fools.
