PHILOSOPHY: LOGIC AND LOGICAL ARGUMENTS 8.2 Forensics October 30, 2014.

PHILOSOPHY: LOGIC AND LOGICAL ARGUMENTS

8.2 Forensics October 30, 2014

Objectives: SWBAT

Identify the basic forms of logical argument

Identify the basic pieces of a logical argument

Construct logical arguments

Introduction to Logic

Logic The science of correctness or incorrectness of

reasoning, the study of the evaluation of arguments

A STATEMENT is a declarative sentence, or a part of a sentence Either true or false The Winter Olympics are in Russia this year, but four

years ago they were in Vancouver, Canada. A PROPOSITION is what is meant by the

statement The idea it expresses

Basic Concepts

An ARGUMENT Collection of statements or propositions,

some of which are intended to provide support or evidence for others

PREMISES Statements or propositions in an argument

that are intended to provide support or evidence

CONCLUSION Statement or proposition for which the

premises provide support

Example arguments

P1: If the President lies to Congress, then he should be impeached

P2: The President lied to Congress C: Therefore, he should be impeached

P1: If everything in the Bible is true, then the world was created in six days

P2: The world was not created in six days C: Therefore, not everything in the Bible is

true.

What makes a good argument? DEDUCTIVELY VALID

Impossible for the conclusion to be false if the premises are true

FACTUALLY CORRECT The premises are true

A SOUND argument is one that is Deductively valid AND Factually correct

Discussion #8

7

Modus Ponens

if you study then you succeed

you studyyou succeed

Aristotle called this modus ponens:

if P then QP

Q

PremisesConclusio

n

(a rule of inference – one of the most important rules)

Argument Form

Whether or not an argument is valid depends on its form

Other valid forms

Multiple modus ponens (MMP)if P then QIf Q the RPTherefore, R

Modus tollens (MT)if P then QNot Q

Therefore, not P

Example arguments

P1: If the President lies to Congress, then he should be impeached

P2: he President lied to Congress C: Therefore, he should be impeached

P1: If everything in the Bible is true, then the world was created in six days

P2: The world was not created in six days C: Therefore, not everything in the Bible is

true.

Common Valid Forms

Hypothetical Syllogism (HS)if P then QIf Q then RTherefore, if P then R

Disjunctive Syllogism (DS)Either P or QNot PTherefore, Q

Constructive Dilemma (CD)Either P or QIf P then RIf Q then RTherefore, R

Invalid Forms

If Hillary Clinton is a Communist Spy, then she supports Obamacare

She supports Obamacare Therefore, she is a communist spy

If JFK jumped off the Eiffel Tower, Then he is dead He did not jump off the Eiffel Tower Therefore he is not dead

If P then QQTherefore, P

If P then QNot PTherefore, not Q

Evaluating Arguments

Does the argument have a valid form? Are the premises true? If the answer is yes to both, then the

argument is sound If the argument is sound then the

conclusion is true

PHILOSOPHY: EVALUATING LOGICAL ARGUMENTS

8.2 Forensics December 4, 2013

Objective

Evaluating arguments and the use of logic in their construction


Logical evaluation Does the argument have a valid form Are the premises true?

If the answer to both questions is “yes,” then the argument is sound and its conclusion is true.


If you think the conclusion is false The form must be invalid The premise must be false

All acts of killing humans are morally wrong

If all acts of killing humans are morally wrong then abortion is morally wrong

Therefore abortion is always morally wrong


You can’t get much out of an unsound argument An unsound argument can still have a true

conclusion

All hamsters are refrigerators All refrigerators are mammals All hamsters are mammals


If God exists, then God created everything in the Universe

If God created everything in the Universe then everything in the Universe is good

If everything in the universe is good, then unnecessary pain and suffering does not exist

Unnecessary pain and suffering does exist

Therefore, God does not exist Be clear – is the ARGUMENT faulty, or is the CONCLUSION false?

The Purpose of Logical Rigor Why so serious?

Most of the issues we will be discussing will be controversial

We need to figure out WHY we disagree when we do

Many of us have attitudes about ethics that are incongruous with one another

The Purpose of Logical Rigor All acts of killing humans is morally

wrong If all acts of killing humans are morally

wrong, then abortion is always morally wrong

Therefore, abortion is always morally wrong

But what about the death penalty?

The Purpose of Logical Rigor Refining the argument

All acts of killing innocent humans are morally wrong

If all acts of killing innocent…. Therefore, abortion is always morally

wrong

PHILOSOPHY: LOGIC AND LOGICAL ARGUMENTS 8.2 Forensics October 30, 2014.

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