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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