1. Summary from Purdue’s Online Writing Lab:
This resource covers using logic within writing-- logical vocabulary,
logical fallacies, and other types of logos-based reasoning.
Contributors:
Ryan Weber, Allen Brizee
Last Edited: 2011-06-28 02:07:33
Website: http://owl.english.purdue.edu/owl/resource/659/1/
2. What is logic?
• Logic is a method of reasoning that helps
prove arguments.
•Logic works by assessing the accuracy of a
collection of statements.
• To be deemed logical, testing of such accuracy
should occur in a sequence.
3. What is logic?
This logical sequence, or syllogism, comes from
the Greek philosopher Aristotle:
Premise 1: All men are mortal.
Premise 2: Socrates is a man.
Conclusion: Therefore, Socrates is mortal.
Note that if Premise 1 is accurate, and Premise 2 is
accurate, a logical conclusion is derived from both
statements.
4. Logical Vocabulary
Before using logic to reach conclusions, it is helpful to know
some important vocabulary related to logic.
Premise: Proposition used as evidence in an argument.
Conclusion: Logical result of the relationship between the
premises. Conclusions serve as the thesis of the argument.
Argument: The assertion of a conclusion based on logical
premises.
Syllogism: The simplest sequence of logical premises and
conclusions, devised by Aristotle.
Enthymeme: A shortened syllogism which omits the first
premise, allowing the audience to fill it in. For example,
"Socrates is mortal because he is a human" is an enthymeme
which leaves out the premise "All humans are mortal."
Induction: A process through which the premises provide some
basis for the conclusion.
Deduction: A process through which the premises provide
conclusive proof for the conclusion.
From “Using Logic”, Purdue OWL
5. Reaching Logical Conclusions
The goal of using syllogisms is to use premises to
arrive at only one logical conclusion.
Example:
Premise 1:
Maria loves sugar.
Premise 2:
Most pastries at the Flying M Coffee Garage
contain sugar.
Conclusion:
Maria loves the pastries at Flying M Coffee
Garage.
6. Reaching Logical Conclusions
• More than 2 premises may be needed to come to a conclusion.
• Logic allows specific conclusions to be drawn from general
premises.
• Logic will not work if the reader does not accept one or more of
the author’s premises.
• From OWL: “Different premises could lead to very different
conclusions about the same subject. For instance, these two
syllogisms about the platypus reveal the limits of logic for
handling ambiguous cases:
Premise 1: All birds lay eggs.
Premise 2: Platypuses lay eggs.
Conclusion: Platypuses are birds.
Premise 1: All mammals have fur.
Premise 2: Platypuses have fur.
Conclusion: Platypuses are mammals.
Though logic is a very powerful argumentative tool and is far
preferable to a disorganized argument, logic does have
limitations. It must also be effectively developed from a syllogism
into a written piece.”
8. Logical Fallacies
Not surprisingly, there are many logical fallacies. Here are some of them to look for and avoid:
Slippery Slope: A conclusion based on the premise that if A happens, and B, C,
D… all the way down the line to Z happens, then A equates to Z.
Hasty Generalization: This happens when a writer jumps to a conclusion based on
insufficient or biased evidence.
Post hoc ergo propter hoc: Assumes that if 'A' occurred after 'B' then 'B' must
have caused 'A.'
Genetic Fallacy: The conclusion draws from the idea that
origins of a determine character, nature, or worth.
Begging the Claim: The conclusion that the writer should prove
is contained and validated within the claim.
Example:
Filthy and polluting coal should be banned.
9. Logical Fallacies
Circular Argument: An argument is restated within the conclusion instead of proving it.
Either/or: This is a conclusion that oversimplifies the argument by reducing it to only two sides
or choices. Example: You can either eat my cooking or starve to death.
Ad hominem: This is an attack on the character of a person rather than her/his opinions or
arguments. Example: Occupy Boise and camps like it are full of homeless thugs.
Ad populum: An emotional appeal that speaks to positive or negative concepts rather than
the real issue at hand. Example: If you were a true blue CWI student you would vote in favor
of the unicorn as school mascot.
Red Herring: This is a diversionary tactic that avoids the key issues,
often by avoiding opposing arguments rather than addressing them.
Straw Man: This move oversimplifies an opponent's viewpoint and then
attacks that hollow argument. Example: People who support the Tea
Party movement want to go back to the days of slavery.
Moral Equivalence: This fallacy compares minor misdeeds with major atrocities.
10. Using Logic in Writing
From OWL:
When converting logical syllogisms into written
arguments, remember to:
• Lay out each premise clearly
• Provide evidence for each premise
• Draw a clear connection to the conclusion.
11. Works Cited
“Purdue OWL: Using Logic." Welcome to
the Purdue University Online Writing Lab
(OWL). Web. 9 Dec. 2011.
