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 PREMISES CONCLUSION
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. Modus Ponens Premises Conclusion Aristotle called this modus ponens:
(a rule of inference – one of the most important rules)
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

8. Argument Form Multiple modus ponens (MMP) Modus tollens (MT)
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 P then Q If P then Q Q Not P Therefore, 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

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. Be clear – is the ARGUMENT faulty, or is the CONCLUSION false?
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