mike mcguire mv community college com 101 a closer look at logos syllogism, enthymeme, and logical...

Mike McGuireMV Community College

COM 101

A CloserLook at Logos

Syllogism, Enthymeme,and Logical Fallacies

ENGL102

OrdoverFall 2008

What is a syllogism?What is a syllogism?

a specific method of logical deduction(moving from the general to the particular)

every syllogism contains at least three parts: a major premise (global assumption) a minor premise (specific claim) a conclusion

It’s kind of like simple math…

If A = B and B = C, then A = C

An example of a syllogismAn example of a syllogism

Socrates is mortal

all men are mortal (major premise)

Socrates is a man (minor premise)

(conclusion)

all things mortalall men are mortal

menSocrates is a man

A visual representationA visual representation

Socrates

Socrates is mortal

An example of a syllogismAn example of a syllogism

fish are not mammals

all mammals have hair

fish do not have hair

(major premise)

(minor premise)

(conclusion)

all things with hairall mammals have hair

mammals


fish do not have hair

fish

fish are not mammals

bad driversAll women are bad drivers.

womanJean is a woman.


Jean

Jean is a bad driver.

This example is for educational purposes and does not reflect the opinions of the instructor nor of The University of Arizona.

What is an enthymeme?What is an enthymeme?

sometimes called a “truncated syllogism”

a syllogism without stating either the major or minor premise (it is implied)

less formal than the syllogism

sometimes more persuasive

An example of an enthymemeAn example of an enthymeme

We cannot trust this man because he has perjured himself in the past.

Enthymemes are often“because” statements.

The syllogism behind this enthymeme…The syllogism behind this enthymeme…

Those who perjure themselvescannot be trusted.

This man perjured himselfin the past.

This man cannot be trusted.

(major premise)

(minor premise)

(conclusion)

Enthymemes are sometimes used to hide the underlying assumption upon which an argument is based.

Find it and challenge it.

Beware. Think Critically.Beware. Think Critically.

What are the unstated assumptions?What are the unstated assumptions?

I failed that course because the instructor didn’t like me.Assumption: The instructor fails students he doesn’t like.

I’m not surprised he made the team. After all, his father is the superintendent of schools.Assumption: The superintendent gives special favors to his

family

If I’d only taken my boss to lunch more often, I could have gotten that raise. Assumption: The boss denies raises to people who don’t take

him to lunch very often.

Mike McGuireMV Community College

COM 101

Logical Fallacies

Avoiding the Pitfalls ofGood Reasoning

Looking at the Negative SpaceLooking at the Negative Space

We can learn much about logic by studying that which is not logical—examples of where logic breaks down, logical fallacies.

What is a logical fallacy?What is a logical fallacy?

mistakes we make in logic when presenting our arguments

• false dilemma• straw man• anonymous authority• prejudicial language

examples

False DilemmaFalse Dilemma

a limited number of options (usually two) is given, while in reality thereare more options.

Either you’re for me or against me.

example

America—love it or leave it.

example

Straw ManStraw Man

the author attacks an argument which is different from, and usually weaker than, the opposition’s best argument.

People who oppose war in Iraq probably just don’t like G.W. Bush. But we want an offensive action against Iraq to protect the world.

example

Anonymous authorityAnonymous authority

the author refers to some source of authority but does not name the source nor explain its legitimacy

Studies show that left-handed people are more intelligent than right-handed people.

example

Prejudicial languagePrejudicial language

the author uses language that attacks a person for having contrary views; this attack may be subtle but shifts the focus away from the issue

All good Americans support the views of the president of the United States. Right-minded people will surely agree with that.

example

True vs. valid argumentsTrue vs. valid arguments

true argument = an argument with a conclusion that people commonly consider to be fact based on their worldly experience or wide-spread belief

valid argument = an argument with a conclusion that logically follows its underlying assumption regardless of whether the assumption is true or not

Don’t let your beliefs or common knowledgeblind you to faulty logic.

Is this true, valid, or both?Is this true, valid, or both?

All vegetables are green.

Beets are vegetables.Therefore, beets are green.

all vegetables

beets

good logic, but a faulty assumption:valid but not true


No human being is immortal.

God is not a human being.Therefore, God is immortal.

all things immortal

humanbeings

God

faulty logic but, according to many people’s beliefs, a true statement: invalid argument, but a true conclusion (according to many people’s beliefs)

plants


All weeds are plants.

A flower is a plant.Therefore, all weeds are flowers.

weedsflowers

Remember, in all valid deductive arguments the conclusion is a necessary consequence of the premises. The conclusion here does not logically follow as a necessary consequence; therefore this argument is invalid.

mike mcguire mv community college com 101 a closer look at logos syllogism, enthymeme, and logical...

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