evaluating deductive reasoning and fallacies
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Chapter 8
Evaluate Deductive Reasoning and Spot Deductive Fallacies
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Deductive Validity and Language
• An argument is deductively valid if there is no possibility, real or imaginable:– Short of changing the very meanings of the
terms and the rules of grammar:– That will make the premises all true and the
conclusion false
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Deductive Validity and Language
• Certain configurations of language form deductively valid argument templates
• Without violating the meanings of the words/grammatical rules of the language:– There is no possible way for the premises all
to be true and the conclusion false
© 2013 Pearson Education, Inc. All rights reserved.
Deductive Validity and Language
• Reasoning Deductively about Declarative Statements– Denying the Consequent– Affirming the Antecedent– Disjunctive Syllogism– Neither, Unless, and Only
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Deductive Validity and Language
• Reasoning Deductively about Classes of Objects– Applying a Generalization– Applying an Exception– The Power of Only
• One of the most interesting words in the language:
© 2013 Pearson Education, Inc. All rights reserved.
Deductive Validity and Language
– Only has the power to change the meaning of a sentence depending on where it is placed
• Classes and Objects
• Reasoning Deductively about Relationships– Natural languages are rich with terms that
describe relationships
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Deductive Validity and Language
– Our understanding of the logical implications of relational terms is part of:
• Our comprehension of language
– Transitivity, Reflexivity, and Identity• Transitivity Relationship• Reflexivity Relationship• Identity Relationship
© 2013 Pearson Education, Inc. All rights reserved.
© 2013 Pearson Education, Inc. All rights reserved.
© 2013 Pearson Education, Inc. All rights reserved.
© 2013 Pearson Education, Inc. All rights reserved.
Fallacies Masquerading as Valid Deductive Arguments
• Just as there are valid argument templates there are fallacious argument templates
• Precision of thought and expression is the key to avoiding these mistakes in: – Our own argument making and also in our
evaluation of the arguments offered to us
© 2013 Pearson Education, Inc. All rights reserved.
Fallacies Masquerading as Valid Deductive Arguments
• Fallacies When Reasoning with Declarative Statements– Affirming the Consequent– Denying the Antecedent– The House M.D. Fallacy
• Fallacies When Reasoning about Classes of Objects
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Fallacies Masquerading as Valid Deductive Arguments
– There are logically correct ways of reasoning about classes of objects & their members:
• There are familiar mistakes we often hear being made
– False Classification• Examples of False Classification seem remarkably
abundant
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Fallacies Masquerading as Valid Deductive Arguments
– Fallacies of Composition and Division• Reasoning about the relationships of parts and
wholes can appear to be deductively valid: – But fail because the attribute that applies to the parts
may not apply to the whole, or vice versa
– Mistaken Identity
© 2013 Pearson Education, Inc. All rights reserved.
Fallacies Masquerading as Valid Deductive Arguments
• Fallacies that occur when reasoning about relationships like identity, reflexivity, or transitivity:
– Most often occur when people think they are talking about the same thing, but in fact are not
– False Reference– Create Your Own Deductive Reasoning
Examples
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