1. Syllogism, Enthymeme, and Logical Fallacies
A Closer Look at Logos
Syllogism, Enthymeme, and Logical Fallacies

2. 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…
If A = B and B = C, then A = C

3. An example of a syllogism
all men are mortal
(major premise)
Socrates is a man
(minor premise)
(conclusion)
Socrates is mortal

4. A visual representation
all things mortal
all men are mortal
men
Socrates is a man
Socrates
Socrates is mortal

5. An example of a syllogism
all mammals have hair
fish do not have hair
(major premise)
(minor premise)
(conclusion)
fish are not mammals

6. A visual representation
all things with hair
all mammals have hair
mammals
fish do not have hair
fish
fish are not mammals

7. 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

8. An example of an enthymeme
We cannot trust this man because he has perjured himself in the past.
Enthymemes are often
“because” statements.

9. The syllogism behind this enthymeme…
Those who perjure themselves
cannot be trusted.
This man perjured himself
in the past.
This man cannot be trusted.
(major premise)
(minor premise)
(conclusion)

10. Find it and challenge it.
Beware. Think Critically.
Enthymemes are sometimes used to hide the underlying assumption upon which an argument is based.
Find it and challenge it.

11. 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.

12. Avoiding the Pitfalls of Good Reasoning
Logical Fallacies
Avoiding the Pitfalls of Good Reasoning

13. 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.

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

15. False Dilemma
a limited number of options (usually two) is given, while in reality there are more options.
Either you’re for me or against me.
example
America—love it or leave it.
example

16. Straw 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

17. Anonymous 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

18. 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
All good Americans support the views of the president of the United States. Right-minded people will surely agree with that.
example

19. 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
Don’t let your beliefs or common knowledge blind you to faulty logic.

20. 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

21. Is this true, valid, or both?
No human being is immortal. God is not a human being. Therefore, God is immortal.
all things immortal
human beings
faulty logic but, according to many people’s beliefs, a true statement: invalid argument, but a true conclusion (according to many people’s beliefs)
God

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