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Fake News: Common Logical Fallacies

Tips and resources to sharpen your critical thinking skills when it comes to facts and false information.

Persuasive Speech

Aristotle proposed three types of speech used to persuade people:

Logos: Logic, Evidence
Ethos: Credibility, Authority
Pathos: Emotion, Feeling

Below are common fallacies used in persuasive speech.

Fallacies of Logic (Logos)

Post hoc ergo propter hoc -- After, therefore because of

This fallacy attempts to create a causal relationship between ideas/events. While there may be evidence to eventually support the belief that the two events are linked, the conclusion that the one must be due to the other simply because it happened afterward is false.

"The Giants lost today because I forgot to wear my cap backward."

Cum hoc ergo propter hoc -- (Along) With, therefore because of

Like the previous fallacy, this one proposes that an idea/event is linked to another idea/event because they happen at the same time. Evidence may eventually support a link, however concurrence is not enough to prove the conclusion.

"There are more people on the train than usual today. It must be because it's so much colder today than the last few days."

Circulus in demonstrando -- Circular argument

In this fallacy, the premise and conclusion are used to support each other in a never ending circle of x because y, and y because x.

"Justice League was a horrible movie because all DCEU movies are horrible. Of course DCEU movies are horrible, look at Justice League."

Dicto Simpliciter -- Sweeping generalization/stereotype

This fallacy proposes a conclusion for all instances of the premise, even though there are most likely to be exceptions.

"You can't trust elves, they're all stuck-up know-it-alls who think they're better than everyone else."

Straw man

This fallacy occurs when an opponent attacks an exaggerated version of your argument rather than your actual argument.

Person 1: "I believe hunting for sport is immoral." Person 2: "Apparently, my opponent wants us all to be vegetarians because animals are more important than people."

Slippery slope

The Slippery slope fallacy happens when one person concludes that an idea/event will cause a series of other ideas/events, usually increasing in severity, without offering evidence as to why the trigger occurs.

"If I let my child play video games then she's not going to do her homework, her grades will suffer, and she won't be able to go to college."

Argumentum ad ad ignorantiam -- Argument to ignorance

This fallacy attempts to prove an idea/event on the grounds that it has yet to be proven false.

"I have yet to hear a reasonable argument against quitting my job and moving to the Alaskan wild, therefore it must be the right choice to make."

Argumentum ad logicam -- Argument to logic

Similar to the previous, this fallacy attempts to prove that an idea/event is false on the grounds that all arguments made so far for the idea have failed. (Even though an argument has failed, there may still one that succeeds)

"There is no picture of Buzz Aldrin on the moon that shows his face, so the moon landing must not have actually happened."

Non sequitur -- It does not follow

In terms of logical argument, a non sequitur is when a conclusion is made from a premise that does not lead to that conclusion, usually because it skips an intermediary step.

Person 1: "I'm wearing a cape, so I'm going to jump off the roof." (Person 2: "Why?" Person 1: "Because Superman wears a cape and he can fly.")

Red Herring

Similar to the previous, this fallacy is an overt attempt to disrupt an opponent's argument by introducing a counter argument that isn't strictly related to the original.

Person 1: "More tax money should be used to build homeless shelters." Person 2: "No, because more homeless would mean more crime."

Naturalistic fallacy

The naturalistic fallacy is an attempt to draw a conclusion from a statement of fact. Like the non sequitur, the naturalist fallacy often skips a necessary premise.

"That book is free. You should take it" (While the book may indeed be free, is that a good enough reason to take a book you know nothing about and may not need?)

Petitio principii -- Begging the question

This fallacy is, perhaps, one of the most incorrectly understood. Begging the question is when the premise of an argument assumes a conclusion of its own in order to justify the final conclusion.

"Sports cars are noisy, therefore they should not be driven after 10pm." (Are all sports cars noisy? Did the speaker check them all?)

Fallacy of Emotion (Pathos)

Argumentum ad misericordim -- Argument (appeal) to pity

This fallacy attempts to prove an argument true because it will benefit and/or stop hurting someone. While this concern is not in and of itself wrong, it cannot be used to simply dismiss any and all objections.

"Programs like that should not be put on television; think of what it does to the children."

Fallacies of Credibility (Ethos)

Argumentum ad hominem -- Argument to (against) the man

This fallacy attempts to disprove an argument by attacking the character of the speaker.

"My opponent wants to increase EPA regulations, but how can you listen to a man who ends sentences with prepositions?"

Argumentum ad verecundiam -- Argument to authority

The opposite to the above, this fallacy attempts to prove an argument because of a misplaced sense of authority.

"Stephen Hawking supports the argument for a higher minimum wage, so that's all I need to know." (While quite brilliant, Prof. Hawking's authority on this subject should not be assumed.)

Argumentum ad antiquitatum --  Argument (appeal) to tradition (That's how we've always done it!)

This fallacy attempts to prove on argument based on what was done in the past and/or done regularly.

"If mailing a hand-written letter was good enough when I was your age, then you don't need those pesky computers."

Argumentum ad populum -- Argument (appeal) to the public

This fallacy attempts to prove an argument is true because the public agrees with it.

"Sanjaya must be a good singer, look at how many people voted to keep him on the show."

Argumentum ad numerum -- Argument (appeal) to numbers

Nearly identical to the above fallacy, this attempts to prove an argument true because of the number of people who agree. The difference between the two fallacies is that the previous is generally used for a smaller, localized group of people.

"52% of people believe ketchup is the best condiment on hot dogs, so you should use it too."

Tu quoque -- You too!

This fallacy attempts to defend the problems with one's own argument by pointing out that the other side made the same mistakes.

"It doesn't matter what Trump has done, look at what Hillary did."

Appeal to nature

This fallacy proposes that if something is natural it is automatically good. However, proponents of this fallacy often either don't define what natural means, or offer questionable definitions.

"Homosexuality is wrong because procreation can only naturally happen between heterosexual couples."