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Chapter 5

ENC 1101 Chapter Notes - Chapter 5: Cyanoacrylate, Apple Inc., Deductive Reasoning


Department
Center for Written and Oral Communication
Course Code
ENC 1101
Professor
Joseph Heizman
Chapter
5

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Logic and Logical Fallacies
Logic enables us to tell whether a conclusion correctly follows from a set of
statements or assumptions
Deductive Reasoning
Premises: statements or assumptions on which an argument is based or from
which conclusions are drawn.
Deductive reasoning moves from premises to specific conclusions
Syllogism: the process of deduction
Major Premise: a general statement that relates two terms
Minor Premise: an example of the statement that was made in the major
premise.
Conclusion: a statement supported by both the two premises.
Ex: Declaration of Independence
Major Premise: When a government oppresses people, the
people have a right to rebel against the government.
Minor Premise: The government of England oppresses the
American people.
Conclusion: Therefore, the American people have the right
to rebel against the government of England.
Valid vs. True syllogisms
Valid: the conclusion follows logically from its premises.
True: the premises are consistent with the facts.
To be sound, the syllogism must be both valid and true.
Valid syllogisms are not always true
Middle Term: the term in a syllogism that occurs in both the major and
minor premises but not in the conclusion.
Illogical middle terms make the syllogism invalid.
Valid syllogisms have a middle terms that refer to all
members of the designated class or group.
Invalid syllogism:
Major Premise: All dogs are mammals.
Minor Premise: Some mammals are
porpoises.
Conclusion: Therefore, some porpoises are
dogs.
Valid syllogism:
Major Premise: All dogs are mammals.
Minor Premise: Ralph is a dog.
Conclusion: Therefore, Ralph is a mammal.
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To be valid, a syllogism’s key terms must stay consistent in meaning
Invalid syllogism:
Major Premise: Only man is capable of analytical reasoning.
Minor Premise: Anna is not a man.
Conclusion: Therefore, Anna is not capable of analytical
reasoning.
If either premise in a syllogism is negative, the conclusion must be
negative.
If both premises are negative, the syllogism cannot have a valid
conclusion
Enthymeme: a syllogism with one or two parts of its argument missing
Usually the major premise
Occurs when the author assumes the reader can infer the rest of
the argument.
Ex: Robert has lied, so he cannot be trusted.
Major Premise: People who lie cannot be trusted.
Minor Premise: Robert has lied.
Conclusion: Therefore, Robert cannot be trusted.
Writing Deductive Arguments:
1. Introduction: Presents an overview of the issue and states the thesis.
2. Body: Presents evidence for points 1, 2, and 3, and refutes arguments
against the thesis.
3. Conclusion: Brings argument to a close and reinforces the theme.
Inductive Reasoning
Begins with specific observations and moves to a general conclusion.
Ways to reach Inductive Conclusions
1. Particular to General: particular pieces of evidence > general conclusion
a. Ex: foggy bathroom mirror, wet floor, water in the bath tub would tell
you that someone just got done taking a bath.
2. General to General: draw a conclusion based on the consistency of the
observations
a. Ex: Apple Corporation has made quality products for a long time,
therefore it will continue to make quality products.
3. General to Particular: draw a conclusion based on what you generally
know to be true
a. Ex: If you have owned several cars made by Ford and they have
been reliable, then you conclude that a Ford Focus will also be a
reliable car.
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