ch. 4 deductive argument

Ch. 4DEDUCTIVE ARGUMENT

Reasoning from the General to the Specific

Deductive Argument

• A specific conclusion is inferred from a series of generalized statements.

• Conclusions are usually indisputable

vs. Inductive Argument (ch3)

• A general conclusion was inferred from several pieces of information

• Conclusions could contain factors of uncertainty

Syllogism

• The most common way of presenting a deductive argument (not found in inductive arguments)

• Contains a major premise, a minor premise, and a conclusion

Categorical Syllogism (Valid)

• Major Premise All A is B.

• Minor Premise C is A,

• Conclusion C is B.

Categorical Syllogism example

MAJ: All students in Critical Thinking are nice.

A is/are B)

MIN: Katie is a student in Critical Thinking.

(C is A)

CON: Katie is nice. (C is B)

Another way of looking at it.

B. Nice People

A. Students in Critical Thinking

C. Katie

Another example

Major Premise: All WHS students (A) are to be in class at 7:25 AM (B).

Minor Premise: Josh (C) is a WHS student (A).

Conclusion: Josh (C) is be in class at 7:25 AM (B).

What would the diagram look like?

__________________

__________________

__________________

Answer

B. People who must be at school at 7:25

A. WHS students

C. Josh

Notice the difference in this one

• Major Premise: *Most people between the ages of 16 and 18 (A) are students (B)

• Minor Premise: Christina (C) is 18 years old (A)

• Conclusion: Christina (C) is *probably a student (B).

*be aware of overstatement

The diagram

• The diagram must change to reflect the syllogism correctly.

A. 16-18 yr olds (remember most, not all)

B. Students

C. Christina

Incorrect/Untruthful Conclusion

• Major Premise: All students (A) are lazy, ignorant individuals (B).

• Minor Premise: Ryan (C) is a student (A)

• Conclusion: Ryan (C) is a lazy and ignorant individual (B)

• Valid in form, but false premise.

Incorrect/Untruthful Conclusion

• Appears valid in form; however, incorrect conclusion.

• False major premiseB. Lazy, IgnorantPeople

A. All Students

C. Ryan

Incorrect/Untruthful Conclusion

• If the premise(s) is false, the conclusion will be untrue.

• Use the Tests of Evidence from ch. 2 to determine the truth of the premises.– Sufficient evidence?– Evidence deliberately omitted?– Conflict with other evidence?– Relevant evidence?– Accurately reported evidence?

INVALID Categorical Syllogisms

• Major Premise:All A is B.

• Minor Premise:C is B.

• Conclusion: C is A.

• Invalid in form

INVALID example

• Major Premise: All basketball players (A) are good runners (B).

• Minor Premise: Mike (C) is a good runner (B).

• Conclusion: Mike (C) is a basketball player (A).

The Diagram

Mike is a good runner, but that doesn’t mean he is a b-ball player.

C. Mike

A. BB Players

B. Good runners

Enthymeme

• A Catagorical syllogism with an unstated premise– contains conclusion– Missing a premise

• Note: Enthymemes are always considered valid!!

Enthymeme example

• These jeans are sure to be in style because they were purchased from the Gap.– Major Premise (unstated): Most jeans

purchased from the Gap are in style.– Minor Premise: These jeans were purchased

from the Gap.– Conclusion: These jeans are (probably) in

style.

In Conclusion: Differences between inductive & deductive reasoning

1. Arguments using inductive reasoning go from specific to general, and it is difficult to arrive at an indisputable conclusion.

2. Deductive reasoning can produce logical conclusions if (a) the syllogism is correctly structured. (b)the premises satisfy the Tests of Evidence