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Logic for Intro

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Michele Merritt

on 5 June 2017

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Transcript of Logic for Intro

Logic and Reasoning
A Few More Strategies
Argument By Analogy
Name that Fallacy!
1. God exists because the bible says so and the bible is true because it was written by God.

2. The death penalty should be permitted. After all, overpopulation is a problem.

3. Teacher, you should give me a better grade on the assignment. If you don't, I will lose my 4.0, not graduate with honors, not get the job I want, and my life will be ruined.

4. Ashtrays and lung cancer are strongly correlated. Therefore, ashtray use causes lung cancer.

5. Having only one penny to your name is better than nothing. Nothing is better than ice cream. Therefore, having only one penny to your name is better than ice cream.
Two Types of Inference
Logic: The Science of Argument
Flaws, mistakes, illegal inferences, defect of reasoning
Writing Effective Argumentative Essays
Ask yourself:
1. What do I want to prove?
2.What kind of argument will I construct??
3. If deductive, does the supposed truth of
my premises necessitate the conclusion?
Are my premises actually true?
4. If inductive, do I have evidence for all
my premises? Does the evidence actually
support the conclusion?
Many, Many ways to go Wrong
Mere Assertion Begging the Question Circular Reasoning
Irrelevancies Ad Hominems Unclear/Changing Meanings (Equivocation or
Amphiboly )
Red Herrings Complex Questions Unqualified Appeal to Authority
Slippery Slope Straw Man Attacks Emotional Appeals
Appeal to Force Misidentification of Cause Post Hoc Ergo Propter Hoc
Defending your Position
1. Counterexamples?
2. Have I committed
any fallacies?
3. Am I being clear
with each and every term
I use?
Philosophical Debate
What is an argument?
Consider this scenario:
Bob and Julie are arguing about the death penalty. Julie says she is in favor of allowing it and cites several reasons, including 1. prisons cost too much money, 2. some crimes are deserving of death, and 3. the world is overpopulated anyways. Bob responds to her argument by saying he disagrees. First of all, he claims, murdering murderers does not solve problems. Second, he cites evidence that it is not in fact the case that imprisoning people costs more than killing them. And third, he tells Julia she can't really have a good argument regarding all of this because she is a dumb blonde.

Take a minute to think about what claims can be supported by good reasoning here and which cannot.
An argument is a group of statements (premises), one or more of which lead up to and purportedly give evidence for accepting a final statement (the conclusion)
Inferential Reasoning
The move from a set of reasons (premises) to a conclusion. Some inferences are warranted. Some are not.
the premises supposedly
necessitate the conclusion
the premises supposedly probably
imply the conclusion
Valid: Premises actually do force you to the conclusion IF you accept the premises
Invalid: Premises do not force you to accept the conclusion, EVEN IF the premises are true
All cats are dogs;
All dogs are fish;
Therefore, all cats are fish
All women are human;
All men are human;
Therefore, all women are men
Strong: The premises probably imply the conclusion; there is strong evidence to accept the conclusion based on the truth of the premises
E.g. I've surveyed 10,000 students at ASU, and my sample of students included a wide range of majors, ages, races, ethnicities, political views, etc. I found that of those students, 80% of them admit to having smoked marijuana at least once. I can conclude, therefore, that a large majority of students at ASU have tried marijuana.
Weak: The premises probably don't imply the conclusion
E.g. I took a philosophy class in college and it was terrible. Therefore, the entire discipline is useless.
Reductio Ad Absurdum
Descartes' Evil Demon:

I doubt I exist
But doubting is thinking
Thinking implies someone is doing the thinking
I am the one doing the thinking
Therefore, I exist.
Can be deductive or inductive
Start with a THESIS - the thing you are trying to prove.

Then list as many reasons as you can think of to support that thesis.
Full transcript