A Closer Look at Logos - University of Arizona

COM 101
Mike McGuire
MV Community College
A Closer
Look at Logos
Syllogism, Enthymeme,
and Logical Fallacies
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 syllogism
all men are mortal
(major premise)
Socrates is a man
(minor premise)
Socrates is mortal
A visual representation
all men are mortal
all things mortal
Socrates is a man
Socrates is mortal
An example of a syllogism
all mammals have hair
(major premise)
fish do not have hair
(minor premise)
fish are not mammals
A visual representation
all mammals have hair
fish do not have hair
all things with hair
fish are not mammals
A visual representation
All women are bad drivers.
bad drivers
Jean is a woman.
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?
 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
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.
Beware. Think Critically.
Enthymemes are sometimes
used to hide the underlying
assumption upon which an
argument is based.
Find it and challenge it.
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
 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.
COM 101
Mike McGuire
MV Community College
Logical Fallacies
Avoiding the Pitfalls of
Good Reasoning
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?
 mistakes we make in logic when presenting our
false dilemma
straw man
anonymous authority
prejudicial language
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.
America—love it or
leave it.
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.
Anonymous authority
 the author refers to some source of authority but
does not name the source nor explain its
Studies show that left-handed
people are more intelligent than
right-handed people.
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.
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.
Is this true, valid, or both?
 All vegetables are green.
Beets are vegetables.
Therefore, beets are green.
all vegetables
good logic, but a
faulty assumption:
valid but not true
Is this true, valid, or both?
 No human being is immortal.
God is not a human being.
Therefore, God is immortal.
all things 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 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.