Pattern
The fallacy of the undistributed middle takes the following form:
- All Zs are Bs
- Y is a B
- Therefore, Y is a Z
This can be graphically represented as follows:
where the premises are in the green box and the conclusion is indicated above them.
Here, B is the middle term, and it is not distributed in the major premise, "all Zs are Bs".
It may or may not be the case that "all Zs are Bs," but this is irrelevant to the conclusion. What is relevant to the conclusion is whether it is true that "all Bs are Zs," which is ignored in the argument. The fallacy is similar to affirming the consequent and denying the antecedent. However, the fallacy may be resolved if the terms are exchanged in either the conclusion or in the first co-premise. Indeed, from the perspective of first-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.
Read more about this topic: Fallacy Of The Undistributed Middle
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—Michel de Montaigne (1533–1592)
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—William Shakespeare (1564–1616)