Evaluating deductive reasoning and fallacies

Chapter 8

Evaluate Deductive Reasoning and Spot Deductive Fallacies

© 2013 Pearson Education, Inc. All rights reserved.

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



• 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



• Reasoning Deductively about Declarative Statements– Denying the Consequent– Affirming the Antecedent– Disjunctive Syllogism– Neither, Unless, and Only



• 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:



– 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



– 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


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



• Fallacies When Reasoning with Declarative Statements– Affirming the Consequent– Denying the Antecedent– The House M.D. Fallacy

• Fallacies When Reasoning about Classes of Objects



– 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



– 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



• 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

Evaluating deductive reasoning and fallacies

Education

Transcript of Evaluating deductive reasoning and fallacies