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Logical Fallacies

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Michael Spencer

on 18 November 2013

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Transcript of Logical Fallacies

Logical Fallacies
Some "booby" traps of logic.
Fallacies of Presumption
Fallacies of Ambiguity
Accent Fallacies
Equivocation Fallacy
Straw Man Fallacy
Slippery Slope
No True Scotsman
Post Hoc Fallacy
Circular Reasoning
Genetic Fallacy
Red Herring
Bandwagon Fallacy
Fallacies of Relevance
Genetic fallacies are committed when an idea is
accepted or rejected based simply on its origins rather than
its merit.
An argument is circular if its conclusion is among its premise; if it assumes what it is trying to prove.
No True Scotsman is a way of reinterpreting the evidence of a claim to prevent refutation.

Dismisses counter examples as non-examples.
After this, therefore because of this.

This fallacy is committed when one presents correlation
as causation.
Slippery slope arguments falsely assume and claim
one thing must lead to another.
Accent fallacies depend on what word(s) are emphasized in a sentence.

Emphasis of certain words suggest importance and meaning.
Fallacies of Equivocation are committed when a term is given two different meanings within the same argument.

Change in the terminology means change in the subject; therefore it is a fallacy.
These fallacies misrepresent someone's argument to
make it easier to discredit and attack.
Fallacies of relevance use irrelevant
logic to support a claim.

1) The origin of the claim is given.
2) Given its origins, the claim is proven or discredited.
Stem cell research was pioneered in
Nazi Germany. Therefore, stem cell
research is bad.
1) Topic A is equated with popularity.
2) Therefore, topic A deemed legitimate.
Formula
Example
Justin Bieber has 10 billion followers on Twitter. Therefore, he must be a
talented singer.
Example
1) Topic A is being discussed.
2) Topic B is introduced as being relevant (but it's not).
3) Topic A is abandoned to discuss B instead.
Formula
Students who cheat in exams should not be punished.
Instead, we should be looking at whether or not the
teacher is doing a good job.
Formula
Fallacies of presumption rest on dubious or unfounded assumptions.
Formula
1) Premise on which the truth of the conclusion is claimed or the truth of the conclusion is assumed.
2) Conclusion must be true.
Example
God exists because it says so in the Bible, and the Bible was written by God.
Formula
Example
Formula
1) A happens.
2) Then B happens.
3) Therefore B is the result of A.
Example
I walked under a ladder then my dog died. Walking under ladders must be bad luck.

The Sun rises after the rooster crows. Therefore, the rooster's crowing must cause the Sun to rise.
Formula
1) If event/thing X has occurred,
2) Then event/thing Y is bound to also occur.
Example
If we let the government ban smoking in public places, then before you know it you won't be able to smoke in your homes.
Fallacies of ambiguity result of ambiguous or vague language that produces confusion or incorrect conclusions.
Example
Two people are discussing a rumour about the behaviour of a 3rd person.

1) I can imagine him doing that, it's possible.
2) Yes, it's possible to imagine him doing that.

Depending on where emphasis is placed, the response can either be of agreement or
disagreement.
Example
1) All jackasses have long ears.
2) Carl is a jackass.
3) Therefore, Carl has long ears.
Example
1) A feather is light.

2) What is light cannot be dark.

3) Therefore feathers cannot be dark.
Formula
1) Person A makes claim X.
2) Person B presents claim Y (which is a distorted version of claim X)
3) Person B attacks claim Y (rather than claim X).
4) Therefore claim X appears to have been falsified or discredited.
Example
Jack: Sunny days are good.

Jill: If all days were sunny we would never have rain, and without rain we would have drought, which means we would have famine and death.
Example
Claim: No "true" Scotsman puts sugar on his
porridge.
Counter: But Angus, a Scotsman, puts sugar
on his porridge.
Claim: Then Angus is not a "true" Scotsman.
A: Claim
B: Counter example
C: Denial of counter example as an actual example.
Red herrings distract audiences from the
argument at hand by pointing to something
else.

This is a tactic of misdirection.
Bandwagon fallacies rest upon the popularity of
an idea.

Rather than looking at the idea itself, bandwagon
appeals point to the general acceptance of the idea as
proof of its validity.
Hitler was not a true socialist, because socialism demands equality and fairness.
Apple controls over half of the cell phone market. Therefore, Apple products are clearly superior.us exists. Therefore, Santa Claus must exist.
Straw Man
People who are in favour of abortion support cold-blooded murder.
People who are in favour of abortion support cold-blooded murder.
A plane is a carpenter’s tool, and a Boeing 737 is a plane, therefore the Boeing 737 is a carpenter’s tool.
After seeing Mission Impossible, I never want to see another Tom Cruise movie.
A warm beer is better than nothing. Nothing is better than a cold beer. Therefore a warm beer is better than cold beer.
My spouse must be cheating on me. He said, “I don’t really love you now.”
A person can quit smoking as long they really want to.
Post Hoc
Every time I watch the news something bad happens. I should stop watching the news so bad stuff stops happening.
Every time I watch the news something bad happens. I should stop watching the news so bad stuff stops happening.
Whether illegally downloading movies is right or wrong is irrelevant. There are – after all – many other more serious crimes that people could be committing.
The logical next step after legalizing abortion is legalizing murder.
Genetic Fallacy
My parents told me that Santa Claus exists. Therefore, Santa Claus must exist.
My parents told me that Santa Claus exists. Therefore, Santa Claus must exist.
Bandwagon Appeal
Apple controls over half of the cell phone market. Therefore, Apple products are clearly superior.us exists. Therefore, Santa Claus must exist.
Equivocation
A plane is a carpenter’s tool, and a Boeing 737 is a plane, therefore the Boeing 737 is a carpenter’s tool.
Accent Fallacy
After seeing Mission Impossible, I never want to see another Tom Cruise movie.
Equivocation
A warm beer is better than nothing. Nothing is better than a cold beer. Therefore a warm beer is better than cold beer.
Accent Fallacy
My spouse must be cheating on me. He said, “I don’t really love you now.”
Circular Logic
A person can quit smoking as long they really want to.
Red Herring
Whether illegally downloading movies is right or wrong is irrelevant. There are – after all – many other more serious crimes that people could be committing.
Slippery Slope
The logical next step after legalizing abortion is legalizing murder.
Hitler was not a true socialist, because socialism demands equality and fairness.
Apple controls over half of the cell phone market. Therefore, Apple products are clearly superior.
People who are in favour of abortion support cold-blooded murder.
A plane is a carpenter’s tool, and a Boeing 737 is a plane, therefore the Boeing 737 is a carpenter’s tool.
After seeing Mission Impossible, I never want to see another Tom Cruise movie.
A warm beer is better than nothing. Nothing is better than a cold beer. Therefore a warm beer is better than cold beer.
My spouse must be cheating on me. He said, “I don’t really love you now.”
We need to be careful about letting students skip class, because before you know it they will be getting into trouble, doing drugs and committing violent crime.
Supporting reductions on military spending means you don’t support the troops!
People generally die with their eyes closed. So if you want to keep living you should avoid closing your eyes.
Mayor Rob Ford must be a good mayor because the majority of Torontonians elected him.
Person A: All Canadians like hockey and beer.

Person B: Well, I’m Canadian and I don’t really like hockey or beer.

Person A: Then you’re not really a Canadian.
The Toronto Maple Leafs are the worst NHL team because they suck.
Genetic Fallacy
David Beckham – one of the world’s best footballers – wears Adidas shoes. Therefore, it’s clear that Adidas shoes are good.
David Beckham – one of the world’s best footballers – wears Adidas shoes. Therefore, it’s clear that Adidas shoes are good.
If you don’t support the Prime Minster’s economic policies, you don’t support the Prime Minister. If you don’t support the Prime Minister, you support the terrorists.
A person can quit smoking as long they really want to.
Post Hoc
Every time I watch the news something bad happens. I should stop watching the news so bad stuff stops happening.
Every time I watch the news something bad happens. I should stop watching the news so bad stuff stops happening.
Whether illegally downloading movies is right or wrong is irrelevant. There are – after all – many other more serious crimes that people could be committing.
The logical next step after legalizing abortion is legalizing murder.
Genetic Fallacy
My parents told me that Santa Claus exists. Therefore, Santa Claus must exist.
My parents told me that Santa Claus exists. Therefore, Santa Claus must exist.
Sample Fallacies
Bandwagon Appeal
Apple controls over half of the cell phone market. Therefore, Apple products are clearly superior.
Equivocation
A plane is a carpenter’s tool, and a Boeing 737 is a plane, therefore the Boeing 737 is a carpenter’s tool.
Accent Fallacy
After seeing Mission Impossible, I never want to see another Tom Cruise movie.
Equivocation
A warm beer is better than nothing. Nothing is better than a cold beer. Therefore a warm beer is better than cold beer.
Accent Fallacy
My spouse must be cheating on me. He said, “I don’t really love you now.”
Slippery Slope
We need to be careful about letting students skip class, because before you know it they will be getting into trouble, doing drugs and committing violent crime.
Straw Man
Supporting reductions on military spending means you don’t support the troops!
Post Hoc
People generally die with their eyes closed. So if you want to keep living you should avoid closing your eyes.
Bandwagon Appeal
Mayor Rob Ford must be a good mayor because the majority of Torontonians elected him.
No True Scotsman
Person A: All Canadians like hockey and beer.

Person B: Well, I’m Canadian and I don’t really like hockey or beer.

Person A: Then you’re not really a Canadian.
Circular Logic
The Toronto Maple Leafs are the worst NHL team because they suck.
Circular Logic
A person can quit smoking as long they really want to.
Slippery Slope
The logical next step after legalizing abortion is legalizing murder.
No True Scotsman
Hitler was not a true socialist, because socialism demands equality and fairness.
Straw Man
People who are in favour of abortion support cold-blooded murder.
Red Herring
If you don’t support the Prime Minster’s economic policies, you don’t support the Prime Minister. If you don’t support the Prime Minister, you support the terrorists.
Red Herring
Whether illegally downloading movies is right or wrong is irrelevant. There are – after all – many other more serious crimes that people could be committing.
Full transcript