# A Closer Look at Logos - University of Arizona

ENGL102 COM Ordover 101 Fall 2008 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 Its 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 (conclusion)

A visual representation all men are mortal Socrates is a man all things mortal men Socrates 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 (conclusion)

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

A visual representation All women are bad drivers. Jean is a woman. bad drivers 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? 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. (conclusion) 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 didnt like me. Assumption: The instructor fails students he doesnt like. Im 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 Id only taken my boss to lunch more often, I could have gotten that raise.

Assumption: The boss denies raises to people who dont 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 arguments

examples 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. example Either youre for me or against me. example Americalove it or leave it. Straw Man the author attacks an argument which is different from, and usually weaker than, the oppositions best argument.

example People who oppose war in Iraq probably just dont 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 legitimacy example Studies show that left-handed people

are more intelligent than righthanded 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 example 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 Dont 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 beets 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. human beings all things immortal God faulty logic but, according to many peoples beliefs, a true statement: invalid argument, but a true

conclusion (according to many peoples beliefs) Is this true, valid, or both? All weeds are plants. A flower is a plant. Therefore, all weeds are flowers. plants weeds 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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