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Looking at Logos. Syllogisms, Enthymemes, and Logical Fallacies. If A = B and B = C, then A = C. 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)

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looking at logos
Looking at Logos




Logical Fallacies

what is a syllogism

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

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…
an example of a syllogism

all men are mortal

(major premise)

(minor premise)

Socrates is a man

An example of a syllogism


Socrates is mortal

a visual representation

all men are mortal

all things mortal


Socrates is a man


Socrates is mortal

A visual representation
an example of a syllogism5

all mammals have hair

(major premise)

(minor premise)

fish do not have hair

An example of a syllogism


fish are not mammals

a visual representation6

all mammals have hair

all things with hair

fish do not have hair



A visual representation

fish are not mammals

a visual representation7

All women are bad drivers.

bad drivers


Jean is a woman.


Jean is a bad driver.

A visual representation

This example is for educational purposes and does not reflect the opinions of the instructor nor of Moraine Valley Community College.

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 enthymeme
An 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

Those who perjure themselves

cannot be trusted.

(major premise)

This man perjured himself

in the past.

(minor premise)


This man cannot be trusted.

The syllogism behind this enthymeme…

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

Find it and challenge it.

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.

logical fallacies
Logical Fallacies

Avoiding the Pitfalls ofGood Reasoning

looking at the negative space
Looking 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


  • false dilemma
  • straw man
  • anonymous authority
  • prejudicial language
What is a logical fallacy?
  • mistakes we make in logic when presenting our arguments
false dilemma



Either you’re for me or against me.

America—love it or leave it.

False Dilemma
  • a limited number of options (usually two) is given, while in reality thereare more options.
straw man


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.

Straw Man
  • the author attacks an argument which is different from, and usually weaker than, the opposition’s best argument.
anonymous authority


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

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


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

Prejudicial 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
true vs valid arguments

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

True 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
is this true valid or both

all vegetables


Is this true, valid, or both?
  • All vegetables are green.Beets are vegetables.Therefore, beets are green.

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

is this true valid or both22


all things immortal

Is this true, valid, or both?
  • No human being is immortal.God is not a human being.Therefore, God is immortal.

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


is this true valid or both23


Is this true, valid, or both?
  • All weeds are plants.A flower is a plant.Therefore, all weeds are flowers.

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.



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