2. Objectives: SWBAT
Identify the basic forms of logical argument
Identify the basic pieces of a logical argument
Construct logical arguments
3. 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
4. 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
5. 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.
6. 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
7. Discussion #8
7
Modus Ponens
if you study then you succeed
you study
you succeed
Aristotle called this modus ponens:
if P then Q
P
Q
Premises
Conclusion
(a rule of inference – one of the most important rules)
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
9. 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.
10. 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
11. 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 Q
Q
Therefore, P
If P then Q
Not P
Therefore, not Q
12. 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
15. Evaluating Arguments
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.
16. Evaluating Arguments
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
17. Evaluating Arguments
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
18. Evaluating Arguments
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?
19. 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
20. 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?
21. 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
