PHILOSOPHY: LOGIC AND
LOGICAL ARGUMENTS
8.2 Forensics October 30, 2014
Objectives: SWBAT
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Identify the basic forms of logical argument
Identify the basic pieces of a logical argument
Construct logical arguments
Introduction to Logic

Logic

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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.

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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
support
or proposition for which the premises provide
Example arguments
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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
Modus Ponens
(a rule of inference – one of the most important rules)
7
if you study then you succeed
you study
you succeed
Premises
Conclusion
Aristotle called this modus ponens:
if P then Q
P
Q
Discussion #8
Argument Form


Whether or not an argument is valid depends on its
form
Other valid forms
Multiple modus ponens
(MMP)
if P then Q
If Q the R
P
Therefore, R
Modus tollens (MT)
if P then Q
Not Q
Therefore, not P
Example arguments

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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 Q
If Q then R
Therefore, if P then R
Disjunctive Syllogism
(DS)
Either P or Q
Not P
Therefore, Q
Constructive Dilemma (CD)
Either P or Q
If P then R
If Q then R
Therefore, R
Invalid Forms
If P then Q
Q
Therefore, P
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If P then Q
Not P
Therefore, not Q
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
Evaluating Arguments
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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
Evaluating Arguments
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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.
Evaluating Arguments
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If you think the conclusion is false
 The
form must be invalid
 The premise must be false
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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
Evaluating Arguments
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You can’t get much out of an unsound argument
 An
 All
unsound argument can still have a true conclusion
hamsters are refrigerators
 All refrigerators are mammals
 All hamsters are mammals
Evaluating Arguments
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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
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But what about the death penalty?
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The Purpose of Logical Rigor
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Refining the argument
All acts of killing innocent humans are morally
wrong
If all acts of killing innocent….
Therefore, abortion is always morally wrong