This document discusses logic and logical arguments. It begins by defining key logical terms like statements, propositions, premises, and conclusions. It then provides examples of valid and invalid argument forms, such as modus ponens, modus tollens, and affirming the consequent. The document explains that for an argument to be sound, it must have a valid logical form and factually true premises. It concludes by discussing how to evaluate arguments by assessing the validity of the form and the truth of the premises.

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

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

13. PHILOSOPHY:
EVALUATING LOGICAL
ARGUMENTS
8.2 Forensics December 4, 2013

14. Objective
 Evaluating arguments and the use of logic in
their construction

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