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Introduction to Logic Class 1: What is Logic?

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Page 1: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Introduction to Logic

Class 1: What is Logic?

Page 2: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

What is Logic?

Definition of Logic: “Logic is the study of virtue in argument, where an

argument is considered virtuous if it helps us get to the truth”

A Contrast:Rhetoric: The study of effective persuasion

Logic: The study of legitimate persuasion

Page 3: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

An argument in logic is not just two people contradicting and

insulting each other.

For more of what an argument is not: http://www.youtube.com/watch?v=kQFKtI6gn9Y

http://www.youtube.com/watch?v=kQFKtI6gn9Y
Page 4: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

DefinitionsStatement: A unit of language that can be

true or false.Argument: A connected series of

statements designed to convince an audience of another sentence.

Conclusion: the statements that an argument tries to convince an audience of.

Premises: the statement that an argument uses to support the conclusion.

Page 5: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

For the purposes of this course, these words will be used interchangeably:

• Sentence• Statement• Assertion• Proposition

They don’t really meant the same thing, but we won’t worry about the difference.

Page 6: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example

1. OJ Simpson intentionally killed Nicole Brown.

2. It is wrong to intentionally kill people.

3. Therefore what OJ did was wrong.

Premise

Premise

Conclusion

Page 7: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Canonical Argument Form

1.Premise 1

2.Premise 2

3.Premise 3

4.Conclusion

Page 8: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

The Study of Argument

Informal LogicThe study of arguments in the real world. It is like a field science. At LCCC, this is covered in Critical Thinking.

Formal LogicThe study of arguments in artificial conditions, including especially invented languages. It is like a laboratory science. At LCCC this is studied in Introduction to Logic.

Page 9: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

An ob/ob mouse and a normal mouse

Via wikipedia, http://en.wikipedia.org/wiki/File:Fatmouse.jpg#file/ . Licensed under Creative Commons.

http://en.wikipedia.org/wiki/File:Fatmouse.jpg
Page 10: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

A normal argument and a formal argument.

“Mortality rates for women undergoing early abortions, where the procedure is legal, appear to be as low as or lower than the rates for normal childbirth. Consequently, any interest of the State in protecting the woman from an inherently hazardous procedure, except when it would be equally dangerous for her to forgo it, has largely disappeared.”

Harry Blackmun, Roe v. Wade

Page 11: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Formal Language

• Formal logic replaces the ordinary language of argument with a symbolic language.

• This language is meant to be free of all ambiguity and vagueness.

• The language is meant to wear its logical structure on its face.

• Our formal languages: SL and QL.

Page 12: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

How to tell an argument1. Look to see if some statements support

others. 2. Look for premises and conclusions

Premise indicator words: because, as, for, since, given that, for the reason that.

Conclusion indicator words: Therefore, thus, hence, so consequently, it follows that, in conclusion, as a result, then, must, accordingly, this implies that, this entails that, we may infer that,

Page 13: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 1

Is this an argument?Cal Ripken has provided years of valuable

service to the Orioles. He has appeared in 19 All-Star games. He was a World Series champion in 1983. His number has been retired by the Orioles. Therefore, he deserves a spot in the Hall of Fame

Example taken from Cathal Woods, Introduction to Reasoning.

Page 14: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 1

Is this an argument?Cal Ripken has provided years of valuable

service to the Orioles. He has appeared in 19 All-Star games. He was a World Series champion in 1983. His number has been retired by the Orioles. Therefore, he deserves a spot in the Hall of Fame

Example taken from Cathal Woods, Introduction to Reasoning.

Page 15: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 1

1. Cal Ripken has provided years of valuable service to the Orioles.

2. He has appeared in 19 All-Star games. 3. He was a World Series champion in 1983. 4. His number has been retired by the Orioles. 5. He deserves a spot in the Hall of Fame

Example taken from Cathal Woods, Introduction to Reasoning.

Page 16: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 2Is this an argument?“We can suspect that the inventor [of eyeglasses]

was not an academic, for professors delight in boasting of their inventions, and before the thirteenth century we have no record by any such self-styled inventor.” —D.J. Boostin, The Discoverers

Example from Salmon, Marilee (1995) Introduction to Logic and Critical Thinking 3rd edition Fort Worth, TX: Harcourt Brace

Page 17: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 2Is this an argument?“We can suspect that the inventor [of eyeglasses]

was not an academic, for professors delight in boasting of their inventions, and before the thirteenth century we have no record by any such self-styled inventor.” —D.J. Boostin, The Discoverers

Page 18: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 21. Professors delight in boasting of their

inventions, 2. Before the thirteenth century we have no

record by any such self-styled inventor.” 3. The inventor [of eyeglasses] was not an

academic.

Page 19: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 3

Is this an argument?

“President Clinton today made a parting appeal to Indians for eased tensions in their region and stronger ties with America as he looked toward a brief and diplomatically dicey stop in Pakistan. ‘Friends don't have to agree on every issue,’ he told business leaders in a domed room of the Bombay stock market. ‘They just have to have an honest relationship about it.’” New York Times March 24, 2000.

Page 20: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 3

Not an argument, just reporting events.

Page 21: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 4Is this an argument?“In England under the blasphemy laws it is illegal to

express disbelief in the Christian religion. It is also illegal to teach what Christ taught on the subject of non-resistance. Therefore, whoever wishes to avoid being a criminal must profess to agree with Christ’s teaching but must avoid saying what that teaching was.”

—Bertrand Russell, Skeptical Essays (1928)

Page 22: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 4Is this an argument?“In England under the blasphemy laws it is illegal to

express disbelief in the Christian religion. It is also illegal to teach what Christ taught on the subject of non-resistance. Therefore, whoever wishes to avoid being a criminal must profess to agree with Christ’s teaching but must avoid saying what that teaching was.”

—Bertrand Russell, Skeptical Essays (1928)

Page 23: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Example 4

1. In England under the blasphemy laws it is illegal to express disbelief in the Christian religion

2. It is also illegal to teach what Christ taught on the subject of non-resistance.

3. Whoever wishes to avoid being a criminal must profess to agree with Christ’s teaching but must avoid saying what that teaching was.

Page 24: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

• Premise• Premise• Conclusion

This motion is inference

Another Definition

Inference: The connection between statements in an argument. Argument glue.

Page 25: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

ValidAn argument is valid if it is impossible for the premises to be true and the conclusion false.

SoundAn argument is sound if it valid and has true premises.

Page 26: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

A Valid Argument

All people are mortal

Socrates is a person.

Socrates is mortal

Page 27: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

Another Valid Argument

All people are carrots

Socrates is a person.

Socrates is carrot

Page 28: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

An invalid argument

All people are mortal

Socrates is a mortal

All people are Socrates

Page 29: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

A Valid Argument

If George Washington were beheaded, he would be dead.

George Washington was beheaded.

Therefore George Washington is dead.

Page 30: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

An Invalid Argument

If George Washington were beheaded, he would be dead.

George Washington is dead.

Therefore George Washington was beheaded.

Page 31: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

StrongAn argument is strong if the premises would make the conclusion more likely if they were true.

CogentAn argument is cogent if it is strong and the premises are true.

Page 32: Introduction to Logic Class 1: What is Logic?. What is Logic? Definition of Logic: “Logic is the study of virtue in argument, where an argument is considered

DeductiveAn argument is deductive if it aims at validity

InductiveAn argument is inductive if it aims at strength


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