Affirming a Disjunct
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(Redirected from Improper Disjunctive Syllogism)
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This page is part of the EvoWiki encyclopedia of fallacies. |
Contents |
Synonyms
- Affirmation of a Disjunct
- Affirming One Disjunct
- Alternative syllogism fallacy
- Asserting an Alternative
- Fallacy of the Alternative Syllogism
- Improper Disjunctive Syllogism
Explanation
You commit this fallacy if you interpret an "inclusive or" as an "exclusive or" like this: "A or B is true; A is true, therefore B is false"
Example
- The bird is alive or on the ground.
- The bird is on the ground.
- Therefore, it is not alive.
The above example is fallacious because both disjuncts can be true - a bird can be both alive and on the ground.
Demarcation
You do not make this fallacy if the 'or' really is exclusive. That is, if A and B really are contrary to each other (i.e. if A and B cannot both be true).
The English language is tricky in this respect, in that "or" can sometimes be used in its inclusive sense, and sometimes its exclusive sense. Which meaning is intended is not always obvious. The difference between inclusive-or and exclusive-or is as follows:
- Inclusive or: (A or B) implies A, or B, or both
- Exclusive or: (A "xor" B) implies A, or B, but not both
