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Deductive Reasoning

Deductive Reasoning. Deductive reasoning moves from a generalization that is true or self-evident to a more specific conclusion.

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Deductive Reasoning

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  1. Deductive Reasoning Deductive reasoning moves from a generalization that is true or self-evident to a more specific conclusion

  2. One kind of deductive reasoning is a syllogism – A syllogism is a three-part set of statements or propositions that includes a major premise, a minor premise and a conclusion. • For Example: • Major Premise: all books from Barnes & Noble are new • Minor Premise: this book is from Barnes & Noble • Conclusion: this book is new

  3. A major premise of a syllogism makes a general statement that the writer believes to be true • The minor premise presents a specific example of the belief that is stated in the major premise • If the reasoning is sound, the conclusion that follows MUST be true because it follows from the premises. • Note that the conclusion contains no terms that have not already appeared in the premises. The strength of a deductive argument is that if readers accept the premises, they must grant the conclusion

  4. Constructing Sound Syllogisms • A syllogism is valid (or logical) when its conclusion follows from its premises. • A syllogism is true when it makes accurate claims (ie. The information it contains is consistent with the facts). • To be sound, a syllogism must be both valid and true. • A syllogism can be valid without being true, or vice versa.

  5. The following syllogism is valid, but not true, and therefore not sound • Major premise: all politicians are male • Minor premise: Wendy Davis is a politician • Conclusion: Therefore, Wendy Davis is male • As odd as it may seem, this syllogism is valid. In the major premise, the phrase all politicians establishes that the entire class politicians is male. After Wendy Davis is identified as a politician, the conclusion that she is male follows. But of course, she is not male. The major premise is not true, therefore no conclusion based on it can possibly be true. Even though the logic is correct, its conclusion is not. Therefore, the syllogism is not sound.

  6. Major premise: All girls with blue eyes go to Central Park on Mondays • Minor premise: Sally has blue eyes • Conclusion: Sally goes to Central Park every Monday

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