Logical Fallacies in Writing

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This page offers a list of common logical fallacies.  But first, what is a logical fallacy?

A logical a fallacy is an inaccurate or intentionally misleading misapplication of logic.

Proper and Improper Use of Logic Words

Yes1) Socrates is a man. All men are mortal. Therefore, Socrates is mortal.
Simple transference. The definition of “man” includes the concept “mortal being”.If you know Socrates fits all the criteria for being a man, then he also fits all the criteria for being mortal. But “Socratese is mortal, all men are mortals, therefore Socrates is a man” is false. (Socrates may be a pet guppy — mortal, but not a man.)
Yes2) The water droplets are too heavy for the atmosphere to support. Thus, they fall to the ground as rain.
Cause-and-effect. Heavy things push light things out of the way and sink. (But did the droplets become heavier, or did the atmosphere change so that it is capable of supporting less weight? This sentence doesn’t answer that question.)
No3) Plants and animals must coexist on earth in order for life to continue.Therefore, plants provide animals with oxygen, and animals provide plants with carbon dioxide.
Yes, plants and animals coexist, yes, life on earth continues. But this example contains an unexamined premise (“life on earth must continue”) that turns out to be a red herring; it works its way into the equation, distracting the reader from the real issue.Life continues because of the arrangement between plants and animals; the plants and animals did not adopt their behavior because they were motivated by the imperative to continue life on earth.
No4) There must be a constant flow of water; thus, nature provides a way for water to get from the end of a stream back to the beginning.
The premise “water must return to the beginning of a stream” is weak, because it begs the question.  By definition a stream is a constant flow of water.  Once the flow stops, you no longer have a stream.  Therefore, if you’ve got a stream, it’s because you’ve got a constant (presumably replenishable) source of water.

We can salvage the two faulty examples, but doing so affects the outcome of the statements:

Yes3A) Plants and animals are symbiotic: therefore, life on earth continues.
YesNature brings water from the end of a river back to the beginning; thus, a stream can flow constantly.

List of Common Logical Fallacies

False Dichotomy
Ad Hominem
Non Sequitur
Hasty Generalization
Circular Reasoning
Red Herring
Loading the Question
Slippery Slope
Post Hoc (Ergo Proper Hoc)
More (links)

False Dichotomy (misuse of either/or)

NoTake it or leave it. (Why not take it for a while, then leave it? Why not do something else — such as change it — before you take it?)
NoIf you aren't part of the solution, you're part of the problem.

Ad Hominem
(“to the person”: an attack directed on the character of an opponent, rather than the issue at hand)
You would say that; you're a crazy liberal tree-hugger.
Applying labels shuts down your thought process too quickly.
Bill Clinton was a bad president because he was an unfaithful husband.
If the topic of your discussion is Clinton’s personal character, this statement might be acceptable — but you would have to construct an argument tying both performance as president and behavior as a husband back to the same character traits.
Fred attacks Clinton's character simply because Fred hates liberals.
If the point of the argument is Clinton’s character, then Fred’s critiques of Clinton are valid (or invalid) regardless of Fred’s character. Someone who attacks Fred is doing so becasue he or she cannot defend Clinton’s character against Fred’s attacks. Resorting to an ad hominem attack is a sign of weakness, like a playground taunt that makes fun of your name.

Non Sequitur
(“does not follow”: a faulty cause-effect relationship)
This car has a noisy engine; therefore, it must be fast.
Because I drink a six-pack a day, the babes will come flocking to me.
Beer companies get rich because lots of young men behave as if this were true!
If you build it, they will come; you did not build it, therefore, they will not come.
To make this work, you would have to prove that “to build it” is the only way to make “them come”.

Hasty Generalization
(from a statistically insufficient or biased sample)
Most of the people in the room opposed the project, so most people in the town probably oppose it, too.
Unless you can prove the sample in the room were statistically representative of the town as a whole, you cannot make this claim.
Nobody at the rally in favor of X said anything about the terrible evil of Y. I guess everyone who supports X must also be in favor of Y.
You can’t use the absence of evidence to prove anything!
Circular Reasoning (self-referential or incomplete definitions; also called begging the question)
NoThe standard written English you will learn in College Writing 101 is defined as the writing style most often employed by college-educated people.
NoA good person is someone who does good things. If I do good things, then I am a good person.

Red Herring
(trying to cloud the main issue with a distraction)
If I hadn't done it, somebody else would have, so it's not really my fault.
I work 60 hours a week to support my family, and I pay my taxes; you shouldn't arrest me just because I punched him in the face.
Begging the Question (building upon a hidden assumption, which may or may not be true)
When did you stop beating your spouse?
Assumptions = 1) you have a spouse 2) you have beaten your spouse 3) the fact that you are not now beating your spouse means that that at some point in the past, you did beat your spouse 4) if you say you never stopped beating your spouse, that means you are still beating your spouse.
Increased police presence on the highways is the best way to stop motorists from abusing toll-booth attendants.
Assumption = The motorists’ behavior towards toll-booth attendants is abusive.
Slippery Slope (extending an argument too far)
First, medicinal wine from a teaspoon... and then, beer from a bottle!  Soon, you'll be an alcoholic.
If we let them ban machine guns and assault weapons, next they'll try to take away our hunting rifles, BB guns and water pistols!  Are we going to stand for that?
Post Hoc (Ergo Propter Hoc) (confusing chronological order with causality)
A flood happened after the comet appeared; therefore, the comet caused the flood.

More Fallacies

Dennis G. Jerz
1998 — first posted
15 Aug 2005 — edits
25 Apr 2011 — minor tweaks

Argument Contents
  1. The Basics: Building an Argument
  2. Components: The Temple Metaphor
  3. Fallacies: Avoid Common Logical Mistakes
  4. Exercises


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