Reason A Way of Knowing

Logic is the beginning of wisdom , Valeris, not the end. - Spock

Introduction

Process of Sherlock Holmes Watchdog barking

Assumptions are called premises

Rationalism declares reason is the most important source of knowledge Rationalists love logic and math,

suspicious of perception

Fallacies are invalid patterns of reasoning

Which is more reliable – reason or perception?

Deductive Reasoning

General to the particular All dogs are mammals Fido is a dog Fido is a mammal

Syllogism – consists of two premises, a conclusion, three terms and quantifiers

First two were truths, third was validity

Argument can be valid while premises and conclusions are not

Exercise in Reasoning

Two true premises and a true conclusion

One true premise, one false premise and a true conclusion

One true premise, one false premise and a false conclusion

Two false premises and a true conclusion

Two false premises and a false conclusion

Deductive Reasoning Preserves Truth

Must be able to say Premises are true Argument is valid

If you begin with truth you will end with it

Enthymeme Argument that is

incomplete because premises are obvious

Exercise in Enthymemes

Fill in the missing premise.

Jenny goes to Oxford University, so she must be very intelligent.

Drugs should be legalized because they only harm the addict.

Graham is a politician so he is probably lying.

Cheerleading should be an Olympic event because cheerleaders compete, train and have a high level of fitness.

Since it is natural to eat meat, there is nothing morally wrong with it.

Discussion Questions

How sure are you that some day you will die? What evidence do you have?

What does it mean to die? What happens? What evidence do you have?

Inductive Reasoning

From the particular to the general Every human I know has died I have never heard of a human that did

not die All human beings die

Allows us to make generalizations about the world Inherited wisdom Example? “You should never generalize” already

a contradiction

How can seemingly obvious generalizations turn out to be wrong? Examples Confirmation bias

Confirmation Bias Activity

Fallacies in Informal Reasoning

Post hoc ergo propter hoc Assuming B is a result of A

because it always follows A In America, as churches increase,

so do prostitutes

Ad hominem Attacking the person not the

argument You are too young to understand

Circular reasoning Assuming truth of something you

are supposed to be proving You can’t me a C because I don’t

get Cs

Fallacies in Informal Reasoning

Special pleading Use of double standards

Watering flowers because they are prize-quality if there is a drought

Equivocation When a word is used in two different sense in an argument

Ad ignorantiam Something is true on the grounds that there is no evidence to disprove it

False analogy Assuming two things are similar in some respects, must be similar in all respects

False dilemma Stating only two options exist when there are actually more

Loaded questions

Exercise in Informal Reasoning

Thinking Outside the BoxExplain the following

What happened here?

A man walks into a bar and asks the barman for a glass of water. The barman pulls out a gun and points it at the man. The man says “Thank you” and walks out.

A man is lying dead in a field. Next to him there is an unopened package. There is no other creature in the field. How did he die?

Anthony and Cleopatra are lying dead on the floor in a villa in Egypt. Nearby is a broken bowl. There is no mark on either of their bodies and they were not poisoned. How did they die?

A man rode into town on Friday. He stayed three nights and then left on Friday. How come?

What happened here?

Two boxers are in a match. The fight is scheduled for 12 rounds but ends after 6 rounds, after one boxer knocks out the other boxer. Yet, no man throws a punch. How is this possible?

In your cellar there are three light switches in the OFF position. Each switch controls one of three light bulbs on the floor above. You may move any of the switches but you may only go upstairs to inspect the bulbs one time. How can you determine the switch for each bulb with one inspection?

What happened here?

A landscaper is given instructions to plant four special trees to that each one is exactly the same distance from the others. How would you arrange the trees?

Connect the nine marks using only four straight lines and without taking your pen off the paper.