Critical Thinking (and Logical Fallacies)

All men are mortal.Socrates is a man.Therefore, Socrates is mortal.

All cadets wear uniforms.Thompson wears a uniform.Therefore, Thompson is a cadet.

All cadets wear uniforms.Thompson is a cadet.Therefore, Thompson wears a uniform.

Mortals

Men

All men are mortal.Lydia is mortal.Therefore, Lydia is a man.

Socrates

Lydia

No cadets lie.Gollum lies.Therefore, Gollum is not a cadet.

People who lie

Cadets

Gollum

Some basketball players are tall.Spud is a basketball player.Therefore, Spud is tall.

People who are tall

Basketball players

Spud?

Spud?

All we can reasonably sayis that Spud might be tall(but he might not be tall).

Some basketball players are tall.Spud is a tall.Therefore, Spud is a basketball player.

People who are tall

Basketball players

Spud?

Spud?

Again, all we can reasonably sayis that Spud might be a

basketball player(but he might not be).

All students study hard.Bob studies hard.Therefore, Bob is student.

No men are immortal.Zeus is immortal.Therefore, Zeus is not a man.

Some men are jerks.Person X is a jerk.Therefore, Person X is a man.

Not all that glitters is gold.Item X glitters.Therefore, item X is not gold.

All men are mortal.Socrates is a man.Therefore, Socrates is mortal.

Syllogism

Major premise (A)Minor premise (B)Conclusion (C)

If the major premise (A) is false, the conclusion is not valid.If the minor premise (B) is false, the conclusion is not valid.If the conclusion (C) does not necessarily follow from A and B, the conclusion is not valid.

All men eat spinach.Socrates is a man.Therefore, Socrates eats spinach.

Syllogism

Major premise (A)Minor premise (B)Conclusion (C)

If the major premise (A) is false, the conclusion is not valid.If the minor premise (B) is false, the conclusion is not valid.If the conclusion (C) does not necessarily follow from A and B, the conclusion is not valid.

All cadets wear uniforms.Obama is a cadet.Therefore, Obama wears a uniform.

Syllogism

Major premise (A)Minor premise (B)Conclusion (C)

If the major premise (A) is false, the conclusion is not valid.If the minor premise (B) is false, the conclusion is not valid.If the conclusion (C) does not necessarily follow from A and B, the conclusion is not valid.

All men are mortal.My dog is mortal.Therefore, my dog is a man.

Syllogism

Major premise (A)Minor premise (B)Conclusion (C)

If the major premise (A) is false, the conclusion is not valid.If the minor premise (B) is false, the conclusion is not valid.If the conclusion (C) does not necessarily follow from A and B, the conclusion is not valid.

Non Sequitur(“does not follow”)

All men are mortal.My dog is mortal.Therefore, my dog is a man.

Faulty PremiseAll cadets wear uniforms.Obama is a cadet.Therefore, Obama wears a uniform.

Ad Hominem(argument against the person)

Joe is a jerk, so Joe’s argument is rubbish.

All jerks use rubbish for argumentsJoe is a jerk.Therefore, Joe’s argument is rubbish.

Major premise is faulty, so the syllogism is faulty.

Post hoc, ergo propter hoc(“after that, therefore because of that”)

When President X was elected,the economy improved.

Therefore, President X’s policies caused the economy to improve.

(Or, “Therefore, because President X was elected,the economy improved.”)

Maybe so – but maybe not. We can’t say for sure.Maybe the economy was already improving.Maybe something else caused it to improve.Timing doesn’t prove causality.

Correlation does not equal Causation

People who read The New Yorker are smart.Therefore, reading The New Yorker makes you smart.

Maybe so – but maybe not. We can’t say for sure.Maybe only really smart people enjoy The New Yorker, so regular people don’t read it.Maybe it’s so challenging that you have to already be really smart to be able to read it.Correlation doesn’t prove causality.

False Analogy

The country is like a big family.Candidate X is a good father,so he would be a good President.

Running the country is like running a business.Candidate X is a successful businessman,so as President he would be successful running the country.

Maybe so – but maybe not. We can’t say for sure.The country is also NOT like a family OR a business, since no family or business has to work through Congress, so neither analogy proves anything.

Begging the Question

Have you stopped beating your wife?Do you still cheat on your taxes?

Wait – the question already assumes a prior condition.

Whether you answer “yes” or “no,” you imply that you HAVE been beating your wife or cheating on your taxes.

False Dichotomy(“either/or” fallacy)

If you don’t support the President, you’re unpatriotic.(Either you support the President, or you’re unpatriotic.)

People who support the President

Patriots

People who support the President

Patriots

Red Herring

The school is already doing plenty to prevent hazing. And have you seen the beautiful new ring on the

corner of the parade ground? It’s a great photo op.

If the issue is hazing, the ring is irrelevant.It is intended to serve only as a distraction – to steer the conversation away from hazing.

Straw Man

Setting up a weak counter-argument, then easily beating it.

It’s sort of like saying that if Southern Cal beats Vandy, then the Pac 12 is better than the SEC,or that if Alabama beats Duke,when the SEC is better than the ACC.Putting a top team from one conference against a weak team from another conference doesn’t prove anything about the two conferences.

The Fallacy Fallacy

Some men are jerks.Person X is a jerk.Therefore, Person X is a man.

The fact that the conclusion doesn’t followdoesn’t necessarily mean that it is not true.

The syllogism might not be valid, butPerson X might still be a man.

These are only a few of the logical fallacies you might spot in other people’s arguments – and that you need to avoid in your own arguments. You can find lists of logical fallacies in most any good English handbook, or even online.