1. Intro to Fallacies
SASP Philosophy

2. What is a Fallacy?
General Definition: a fallacy is an arguments of a type that is generally recognized to be bad
It’s a “bad argument”
It’s an argument with certain features that characterize it as a type

3. What is a Fallacy? Example:
“If John exercises every day and watches what he eats, then he’ll lose weight. John lost weight, so he must be exercising every day and watching what he eats”.
Weak logic – losing weight doesn’t mean that he exercised and watched his eating, he could have only watched his food or lost weight from disease.

4. What is a Fallacy?
What makes this a fallacy is that it’s a bad argument of a certain type: it commits the fallacy of “affirming the consequent”
If A then B B
Therefore, A

5. What is a Fallacy?
If John exercises every day and watches what he eats, then he’ll loose weight
John lost weight
Therefore, John exercised every day and watched what he ate.
“Affirming the consequent” is an example of a “formal” or “structural” fallacy, because the argument form is invalid
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6. What is a Fallacy?
“Denying the antecedent” is another example of a “formal” or “structural” fallacy that creates an invalid argument
If John exercises every day and watches what he eats, then he’ll loose weight
John did not exercise every day or watched what he ate
Therefore, John did not loose weight
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7. What is a Fallacy? But not all fallacies are formal in this way.
Example:
“It’s okay to lie on your taxes. Everyone does it.”
This is a bad argument, but it’s not bad because of its structural form. It’s bad because it relies on an assumed premise that most of us would reject

8. What is a Fallacy?
Here’s how you might reconstruct this argument, filling in the natural assumed premise:
If everyone does something, then it’s okay to do it.
Everyone lies on their taxes.
Therefore, it’s okay to lie on your taxes.
The logic is perfectly valid, the problem is that most people would reject their first assumed premise. The fact that many people do something doesn’t make it morally right. What about slavery?

9. What is a Fallacy?
This fallacy is often called a “bandwagon” fallacy or “the appeal to common practice”
It’s an example of what’s usually called a content fallacy: this is a fallacy that relies on a false or dubious premise of a certain kind.
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10. What is a Fallacy?
You can’t judge an argument based on its superficial form as given, since we often leave out parts and rely on our audience to fill the gaps
Example: “Whales can’t breath underwater. They’re mammals”
It would be unfair to call this argument weak since the connection between being a mammal and breathing underwater isn’t stated:
Whales are mammals
Therefore, whales can’t breathe underwater

11. What is a Fallacy?
We need to make explicit all the implicit premises in the statement before we can decide if it commits a fallacy or not:
Whales are mammals
Mammals can’t breathe underwater
If a mammal spends time underwater, it must come to the surface to breathe
Therefore, whales can’t breathe underwater

12. Categorizing Fallacies
Fallacies are often categorized into different groups or families:
Formal (structural) Fallacies
Ex. Affirming the consequent, denying the antecedent
Informal (content) Fallacies
Bandwagon fallacy, Red Herring, Begging the Question
Kahn Academy
Fallacy Files

13. Some Rules for rational argumentation
You can’t argue with someone who is intentionally trying to mislead of deceive you
You can’t argue with someone who is unwilling to reason well
You can’t argue with someone who is unable to reason well
An argument has to give reasons for believing or accepting the conclusion

14. Summary
A fallacy is an argument of a type that is generally recognized to be bad
You should evaluate an argument only after the argument has been reconstructed to include any implicit or assumed premises. Very often, the fallacy only becomes obvious after you’re reconstructed the reasoning.
Fallacies are generally categorized into formal (structural) and informal (content) fallacies