Affirming a disjunct: Difference between revisions
SolomonFreer (talk | contribs) →Example: Added a useful third option which is more relatable, as the first proposition is actually something that might be proposed in a real life scenario as true |
SolomonFreer (talk | contribs) →Example: Added a useful third option which is more relatable, as the first proposition is actually something that might be proposed in a real life scenario as true |
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Affirming the disjunct should not be confused with the valid argument known as the [[disjunctive syllogism]]. |
Affirming the disjunct should not be confused with the valid argument known as the [[disjunctive syllogism]]. |
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==Example== |
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The following argument indicates the invalidity of affirming a disjunct: |
The following argument indicates the invalidity of affirming a disjunct: |
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The above example of an argument also demonstrates the fallacy. |
The above example of an argument also demonstrates the fallacy. |
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A third example demonstrates a more realistic first proposition, yet still yields a clearly fallacious result. |
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:To be on the cover of Vogue Magazine, one must be a celebrity or very beautiful. |
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:This month's cover was a celebrity. |
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:Therefore, this celebrity is not very beautiful. |
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==See also== |
==See also== |
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Revision as of 05:47, 13 May 2014
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The formal fallacy of affirming a disjunct also known as the fallacy of the alternative disjunct or a false exclusionary disjunct occurs when a deductive argument takes the following logical form:
- A or B
- A
- Therefore, it is not the case that B
Or in logical operators:
- ¬
Where denotes a logical assertion.
Explanation
The fallacy lies in concluding that one disjunct must be false because the other disjunct is true; in fact they may both be true because "or" is defined inclusively rather than exclusively. It is a fallacy of equivocation between the operations OR and XOR.
Affirming the disjunct should not be confused with the valid argument known as the disjunctive syllogism.
Example
The following argument indicates the invalidity of affirming a disjunct:
- Max is a cat or Max is a mammal.
- Max is a cat.
- Therefore, Max is not a mammal.
This inference is invalid. If Max is a cat then Max is also a mammal. (Remember "or" is defined in an inclusive sense not an exclusive sense.)
- The car is red or the car is large.
- The car is red.
- Therefore, the car is not large.
The above example of an argument also demonstrates the fallacy.
See also
External links
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