LOGIC

A Very Short Introduction

Words

We need to define words!

Example:

Jack: “I love you.”Mary: “Wow!”

Mary thinks: Love means staying forever and getting married!

Jack thinks: Love means I’m really attracted to you at this particular moment.

Awkward.

DEFINE LOVE

Some say there are four different kinds…

1. Storge – liking2. Philia –friendship (shared interests)3. Eros – romantic desire4. Agape / Caritas – self-sacrificing

love (desires the good of the other)All are forms of wanting.

Step 1: InductionInduction starts with experience.

• Over the years, I meet a couple dozen people from Omaha and they are in every case extremely nice and kind.

• I call my friend and she doesn’t answer three times.• I didn’t think I would like books by Charles Dickens, but it turns

out that I really enjoyed David Copperfield and Great Expectations.

• I thought everybody from the South was polite, but then somebody with a Southern accent rammed me with her grocery cart.

Exercise 1:What is something you learned or concluded from repeated

experiences?

Repeated experiences lead to

INDUCTIVE LEAPS

Step 2: Forming Premises

• ALL people from Omaha are nice.• NO phone-calls to my friend get

answered.• SOME of Dickens’ books are

enjoyable.• SOME people are NOT polite.

Exercise 2: Express what you learned from repeated experiences as a simple statement.

A note about universals(all-statements):

If it’s not inherent to the thing, it could be otherwise:

For example: All squirrels are brown.There’s nothing about a squirrel that it

means it HAS to be brown.When it is inherent, that’s different.For example: All material things have a

gravitational pull.Gravitational pull is an intrinsic property of

matter.

DEDUCTION

Which now leads us to Step 3 in the reasoning process, namely…

Deductive Reasoning

Based on propositions derived from our experiences, we come to conclusions.

For example:All humans are mortal.

I am a human.Therefore, I am mortal.

The Pieces of Syllogisms

3 TERMSMinor term, Middle Term, Major Term

(Each appears twice.)

2 PREMISES (i.e., syllogistic propositions)Major premise contains the major term and the middle

term.Minor premise contains the minor term and the middle

term.

1 CONCLUSIONWhich joins the major and the minor terms.

Example Syllogism 1

Major Premise: All humans are risible.

Minor Premise: X is a human.Conclusion: Therefore, X is

risible.

Middle Term = “human”Major Term = “risible” (able to laugh)

Minor Term = “X”

Truth & Validity

• Is the conclusion of that syllogism VALID? Beings

that laugh

Humans

X

Is it TRUE?

Do we need a syllogism?

Logic can be somewhat like grammar:1. It reveals what we are already doing.2. As knowing about language can make

us better at using language, knowing about logic can make us better thinkers.

Thus: Onward!

Truth vs. Validity

Truththe correspondence of thought to

reality.

Validitywhether or not the syllogism holds.

Let’s look at some more examples.

Analyze these syllogisms:

Some men are dishonest.Joe is a man.

Therefore, Joe is dishonest

All girls are kind.Mary is a girl.

Therefore, Mary is kind

Enthymeme

An enthymeme is a rhetorical syllogism. It’s abbreviated, and premises are left out.

Example: “She’s coming from SCA, so she’ll be really nice.”

What is the implicit syllogism?

The implicit syllogism:

All SCA girls are nice.She is an SCA girl----------------------------------------------Thus, she is nice.

Questions?

End of PPT.