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Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

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Page 1: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard
Page 2: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Philosophy 103Linguistics 103

Yet, still, even further more, expanded,

Introductory Logic: Critical Thinking

Dr. Robert Barnard

Page 3: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Last Time:• Definitions

– Lexical– Theoretical– Precising– Pursuasive

• Logical Form• Form and Validity

Page 4: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Plan for Today

• Deductive Argument Forms• Formal Fallacies• Counter-Example Construction

Page 5: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Validity and Form• Deductive Validity – IF the premises are true

THEN the conclusion MUST be true.• Deductive Soundness – the deductive

argument is valid AND premises are all true• Form - The structure of an argument. Validity

is a Property of Form.

Page 6: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Common Deductive Logical Forms

• Modus Ponens• Modus Tollens• Disjunctive Syllogism• Hypothetical Syllogism• Reductio Ad Absurdum

Page 7: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Common Logical Forms

• Modus Ponens

If P then Q, P --- Therefore Q

• Modus Tollens

If P then Q, Q is false --- Therefore P is false

Page 8: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Modus Ponens Example

If P then Q, P --- Therefore Q

If Peter is from Ohio then Peter is an AmericanPeter is from Ohio --- Therefore Peter is an American.

Page 9: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Modus Tollens ExampleIf P then QQ is false

Therefore P is false

If Paul is a potter then Paul has worked with clay Paul has not worked with clay. Therefore Paul is not a potter.

Page 10: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Common Logical Forms• Disjunctive Syllogism P or Q, P is false --- Therefore Q

• Hypothetical Syllogism

If P then Q , If Q then R --- Therefore If P then R

Page 11: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Disjunctive Syllogism Example

P or QP is false

Therefore Q

Pizza is yummy or Quiche is manly.Pizza is not yummy.Therefore Quiche is manly.

Page 12: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Inclusive OR vs Exclusive OR

Assume: Tom is a Lawyer or Tom is a Doctor

If Tom is a Lawyer does that require that he is not a Doctor?

Inclusive-OR: No - (Lawyer and/or Doctor)

Exclusive- OR: Yes - ( Either doctor or lawyer, not both)

Page 13: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Hypothetical Syllogism Example

If P then QIf Q then R

Therefore If P then R

If Pigs fly then Cows kiss.If Cows kiss then Otters sing.Therefore If Pigs fly then Otters sing

Page 14: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Common Forms• Reductio Ad Absurdum

(Reduces to Absurdity)

a) Assume that Pb) On the basis of the assumption if you can prove ANY contradiction, then you may infer that P is false

Case of : Thales and Anaximander

Page 15: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Thales and Anaximander

• Arché - Table of Elements

- Thales: Water - Anaximander: Aperion

Page 16: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

The Presocratic Reductio1. Everything is Water (Thales’ Assumption)2. If everything is water then the universe contains an

infinite amount of water and nothing else. (From 1)3. If there is more water than fire in a place, then the

water extinguishes the fire. (observed truth)4. We observe fire. (observed truth)5. Where we observe fire there must be more fire than

water. (from 3 & 4)6. Therefore, everything is water and something is not

water (Contradiction from 5 and 1)7. Thus, (1) is false.

Page 17: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Common Formal Fallacies

• Affirming the Consequent• Denying the Antecedent• Illicit Hypothetical Syllogism• Illicit Disjunctive Syllogism

Page 18: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Common Formal Fallacies

• Affirming The Consequent

If P then Q, Q --- Therefore P

• Denying the Antecedent

If P then Q, P is false --- Therefore Q is false

Page 19: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Affirming the Consequent

If P then QQ is true

Therefore P

1. If it rained last night then the grass is wet

2. The grass is wet.3. Therefore, it rained last night.

Page 20: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Denying the Antecedent

If P then QP is false

Therefore Q is false1. If Tom is not hungry then Tom ate lunch2. Tom is Hungry3. Therefore Tom did not eat lunch.

Page 21: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Common Formal Fallacies

• Illicit Disjunctive Syllogism -P or Q, P is true -- Therefore not-Q

-P or Q, Q is true -- Therefore not-P• Illicit Hypothetical Syllogism(*)

If P then not-Q , If Q then not-R --- Therefore If P then not-R

* - there is more than one form of IHS

Page 22: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Illicit Disjunctive Syllogism

P or QP is true

Therefore not-Q

John is Tim’s father or Sally is Tim’s mother

John is Tim’s FatherTherefore Sally is not Tim’s mother

Page 23: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Illicit Hypothetical Syllogism

If P then not-QIf Q then not-R

Therefore If P then not-R

1. If I like fish then I won’t eat beef2. If I eat beef then I won’t eat cheese3. Therefore, If I like fish then I won’t eat

cheese.

Page 24: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Testing for Validity

The central question we ask in deductive logic is this: IS THIS ARGUMENT VALID?

To answer this question we can try several strategies (including):

a)Counter-example (proof of invalidity)b)Formal Analysis

Page 25: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Counter-Example Test for Validity

1) Start with a given argument2) Determine its form

(Important to do correctly – best to isolate conclusion first)

3) Formulate another argument:a) With the same formb) with true premisesc) with a false conclusion.

Page 26: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

An example counter-example…

1. If Lincoln was shot, then Lincoln is dead.

2. Lincoln is dead.3. Therefore, Lincoln was

shot.

The FORM IS:1. If Lincoln was shot,

then Lincoln is dead.2. Lincoln is dead.3. Therefore, Lincoln was

shot.

1. IF --P-- , THEN --Q--.

2. --Q--

3.Therefore -- P--

Page 27: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

NEXT: We go from FORM back to ARGUMENT…

1. IF --P-- , THEN --Q--.

2. --Q--3. Therefore -- P--

1. IF Ed passes Phil 101, then Ed has perfect attendance.

2. Ed has perfect attendance.

3. Therefore, Ed Passes Phil 101

Page 28: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

NO WAY!

Ed’s Perfect Attendance does NOT make it necessary that Ed pass PHIL 101.

SO: Even if it is true that 1. IF Ed passes Phil 101, then Ed has perfect

attendance.

2. ..AND that..Ed has perfect attendance.

Page 29: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

IT DOES NOT FOLLOW THAT ED MUST PASS PHIL 101!

It is possible to have perfect attendance and not pass

•It is also possible to pass and have imperfect attendance

This shows that the original LINCOLN argument is INVALID.

Page 30: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

This is ED…

Page 31: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Another Example?1. All fruit have seeds2. All plants have seeds3. Therefore, all fruit are plants

Form:All F are SAll P are STherefore All F are P

Page 32: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Another example….cont.Form:All F are SAll P are STherefore All F are P

1.All Balls (F) are round (S).

2.All Planets (P) are round (S).

3. Therefore, All Balls (F)are (P)lanets.

Page 33: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard

Formal Evaluation?

The counter-example test for validity has limits.• Counter-Examples should be obvious.• Our ability to construct an Counter-Example is

limited by our concepts and imagination.• Every invalid argument has a possible counter-

example, but no human may be able to find it.

Page 34: Philosophy 103 Linguistics 103 Yet, still, even further more, expanded, Introductory Logic: Critical Thinking Dr. Robert Barnard