1. Chapter 8
Evaluate Deductive
Reasoning and Spot
Deductive Fallacies

2. 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
© 2013 Pearson Education, Inc. All

3. 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

4. Deductive Validity and Language
• Reasoning Deductively about Declarative
Statements
– Denying the Consequent
– Affirming the Antecedent
– Disjunctive Syllogism
– Neither, Unless, and Only
© 2013 Pearson Education, Inc. All

5. 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

6. 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
© 2013 Pearson Education, Inc. All

7. 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

8. © 2013 Pearson Education, Inc. All

9. © 2013 Pearson Education, Inc. All

10. © 2013 Pearson Education, Inc. All

11. 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

12. 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
© 2013 Pearson Education, Inc. All

13. 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
© 2013 Pearson Education, Inc. All

14. 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

15. 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
© 2013 Pearson Education, Inc. All