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Where differing definitions of the same word are used at different points in the same argument without acknowledging or highlighting this difference, the definitional fallacy is committed.

For example:

P1: Mercury may be found inside most drugstore thermometers.

This is a verifiable statement which any pharmacist ought to be able to confirm for you.

P2: A thermometer must be large enough to contain whatever is inside of it.

This is a commonsensical statement, a tautology which is really not possible to logically dispute.

P3: Mercury has a diameter of over 2400 kilometers.

Any astronomer ought to be able to verify this statement for you.

C1: Most drugstore thermometers are large enough to contain an object with a diameter of over 2400 kilometers.

Obviously the conclusion here is false, and is false because the argument failed to distinguish between the element, mercury, and the planet, Mercury. But typically this fallacy occurs where the difference between meanings of the multidefinitional word is much more subtle, such as between "good" meaning fit for consumption and "good" meaning morally commendable.

The typical occurrence of this fallacy will not come in formal logic, but in informal argument, often where one participant seeks to persuade the other to a position by a combination of appeals to authority and appeals to emotion, by first obtaining agreement to an idea presented as authoritative and then switching definitions of a term in that idea to one with emotional weight.

Naturally, this sort of juxtaposition serves as well (nonargumentatively) as a source of humor, as in: How do you stop a rhino from charging? Take way his credit card!!

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Around 269 words for BrevityQuest11

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