Question

Spell out a reductio ad absurdum argument to refute each of the following claims. If no such reductio is possible, explain why. 1. Some sisters are nephews. 2. Some fathers were never children. 3. Most students scored better than the median grade on the last test. 4. Almost everyone in this class is exceptional. 5. There is an exception to every universal claim. 6. I know that I do not know anything. 7. Some morally wrong actions are morally permitted. 8. God exists outside of time, and we will meet Him someday. 9. There is a male barber in this town who shaves all and only the men in this town who do not shave themselves. (Hint: Does he shave himself?) 10. Most of the sentences in this exercise are true.

   Spell out a reductio ad absurdum argument to refute each of the following claims. If no such reductio is possible, explain why.
1. Some sisters are nephews.
2. Some fathers were never children.
3. Most students scored better than the median grade on the last test.
4. Almost everyone in this class is exceptional.
5. There is an exception to every universal claim.
6. I know that I do not know anything.
7. Some morally wrong actions are morally permitted.
8. God exists outside of time, and we will meet Him someday.
9. There is a male barber in this town who shaves all and only the men in this town who do not shave themselves. (Hint: Does he shave himself?)
10. Most of the sentences in this exercise are true.
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Cengage Advantage Books: Understanding Arguments: An Introduction to Informal Logic
Cengage Advantage Books: Understanding Arguments: An Introduction to Informal Logic
Walter… 9th Edition
Chapter 17, Problem 4 ↓

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A sister is a female sibling, while a nephew is the son of one's sibling.  Show more…

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Spell out a reductio ad absurdum argument to refute each of the following claims. If no such reductio is possible, explain why. 1. Some sisters are nephews. 2. Some fathers were never children. 3. Most students scored better than the median grade on the last test. 4. Almost everyone in this class is exceptional. 5. There is an exception to every universal claim. 6. I know that I do not know anything. 7. Some morally wrong actions are morally permitted. 8. God exists outside of time, and we will meet Him someday. 9. There is a male barber in this town who shaves all and only the men in this town who do not shave themselves. (Hint: Does he shave himself?) 10. Most of the sentences in this exercise are true.
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Key Concepts

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Reductio ad absurdum
A method of argumentation in logic where one assumes the truth of a proposition and then shows that this assumption leads to a logical contradiction. Once the contradiction is derived, the original proposition is refuted because a statement cannot be both true and false.
Proof by Contradiction
This is a specific application of reductio ad absurdum where one assumes the opposite of what one wishes to prove, and through a series of logical deductions, arrives at an impossibility. This inconsistency implies that the original assumption must be false, thereby validating the intended conclusion.
Logical Contradiction
In logical analysis, a contradiction occurs when a set of premises or a singular proposition implies mutually exclusive outcomes that cannot both be true simultaneously. Detecting such contradictions is central to demonstrating the invalidity of certain assertions in a reductio ad absurdum argument.
Logical Quantifiers
These are symbols or words used in logic to express the quantities of specimens in the domain of discourse, such as 'all', 'some', or 'most'. Understanding how universal and existential quantifiers function is crucial when analyzing statements that involve categorical claims or generalizations.
Self-Reference and Paradox
Some logical claims involve self-referential statements, where a statement or a set includes or refers to itself, often leading to paradoxes. Recognizing the problems that arise with self-referentiality, such as in well-known paradoxes, is important to understand why certain arguments fail or become inconsistent.

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