Logical fallacy

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A logical fallacy is a debate trick that attempts to win an argument using an invalid "if-then" statement.

A description of common logical fallacies follows.

Evidence Fallacies

Slippery Slope
Extrapolating progressively larger effects from one cause (i.e. A to B, B to C, C to D, therefore A to D). Only a valid fallacy if you cannot show A causes B, B causes C, etc.
  • Example: "A slice of pizza contains fat. Fat can lead to excessive cholesterol. Cholesterol can lead to clogged arteries. Clogged arteries can lead to a heart attack. Heart attacks can be a cause of death. Thus, a single slice of pizza can kill you."
Confusing fact with opinion
Asserting opinions as fact, or discounting facts as opinions
Red Herring
Distracting readers with irrelevant material
Myth of the Mean
Using averages to cover up problems or to skew perception
Flawed statistical comparisons
Using inappropriate statistics to skew perception or distort a reader's view of reality
  • Example: There were less car accidents per year in the 1920's than there are now, therefore drivers must have been better than today.
Defective testimony
Quoting out of context or omitting a speaker's credentials.

Flawed Proofs

Ad hominem
Attempting to tie the validity of your opponent's argument to his personal credibility rather than its intrinsic merits
Begging the question
The argument fails to prove anything because it applies what it is supposed to prove as fact. This is also known as circular logic.

Defective Arguments

Shaky principle
Basing a line of argument on an unsound assumption.
Omitted qualifiers
Confusing probability with certainty by asserting a conclusion without qualification.
Post Hoc
A occurred before B, thus A caused B
Non sequitur
Reasoning in which principles and observations are unrelated to each other or to the conclusion drawn.
Hasty generalization
Drawing a conclusion from a limited observational pool, or conclusion based on insufficient or non representative observations.
Faulty analogy
An analogy which does not apply to the situation in question

Persuasive Design Fallacies

False Dilemma
Forcing your opponent to choose between two artificially designed choices. "How can you say the attempt at rehabilitation of criminals has merit when so many participants return to prison later?" The choice is between "The rehabilitation of criminals is 100% effective" and "the attempt at rehabilitation of criminals is worthless".
A distortion of your enemy's position so you can knock it down more easily
Appeal to Authority
Using the agreement of a famous individual with an argument as support for that argument. This trick really amounts to little more than name-dropping. The named individual may not have any relevant training or knowledge that would make his or her opinion valuable on the subject. Even so, if the famous individual has good reasons for agreeing with a given claim, then the debator should be citing those reasons, not the agreement of the person.

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