Formal

Formal fallacies are fallacies contained in the form or structure of an argument.

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Square logic:

Formal Fallacy
Definition Example
A complex argument which loses logical cohesion somewhere in the complexity. If you only asked me, I would tell you the truth. But you didn’t ask, so if I told you a lie you wouldn’t believe it was a lie since the truth is not what you asked for.
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Negative proof fallacy:

Formal Fallacy
Definition Example
When it is claimed or implied that, because a premise cannot be proven false, the premise must be true, or that, because a premise cannot be proven true, the premise must be false. Can you prove I’m not wearing an invisible hat? If you can’t, I’m justified in believing I am.
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Illicit major:

Formal Fallacy
Definition Example
A categorical syllogism that is invalid because its major term is undistributed in the major premise but distributed in the conclusion. Because all pigs like mud, and no birds are pigs, therefore no birds like mud.
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Undistributed middle:

Formal Fallacy
Definition Example
When the middle general term in a categorical syllogism is not distributed to more specific terms of the same category. All humans are 2-legged, and all birds are 2-legged. Therefore all humans are birds.
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Exclusive premises:

Formal Fallacy
Definition Example
When a categorical syllogism that is invalid because both of its premises are negative. No humans are birds. Some birds are not lawyers. Therefore some lawyers are not humans.
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Denying the antecedent:

Formal Fallacy
Definition Example
When the consequent in an indicative conditional is claimed to be false because the antecedent is false; if A, then B; not A, therefore not B. If I were a movie star, I’d be popular. But I’m not a movie star, so I’m not popular.
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Begging the question:

Formal Fallacy
Definition Example
When the conclusion of an argument is implicitly or explicitly assumed in one of the premises. This medicine will definitely make you well since it can cure your disease.
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Affirming the consequent:

Formal Fallacy
Definition Example
When the antecedent in a conditional is claimed to be true because the consequent is true; if A, then B; B, therefore A. Whenever it rains, people carry umbrellas. People are carrying umbrellas, so it must be raining.
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Affirming a disjunct:

Formal Fallacy
Definition Example
When it is claimed that one logical disjunction must be false because the other side of the disjunct is true; A or B; A; therefore not B. My new neighbor looked so happy, I knew he had either a girlfriend or a wife. He told me he has a wife, so I know he does not have a girlfriend.
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Affirmative conclusion from a negative premise:

Formal Fallacy
Definition Example
When a categorical syllogism has a positive conclusion, but at least one negative premise. I don’t eat meat. People who eat meat have no compassion. Therefore, I have compassion.
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Association fallacy:

Definition Example
When a claim is deemed true or untrue based on an association with some irrelevant element. Jane is an exceptional pianist, and also pretty. Therefore, all pianists are pretty.
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Inconsistent comparison:

Definition Example
When complex nodes of comparison are used that make it falsely appear as if a complete comparison has been made. My son Arthur runs faster than Thomas, gets higher grades than George, and plays piano better than William. He’s definitely the most talented boy.
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