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.
- 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.
- 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.
- 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.
- Bare Assertion
- This is just simply making a claim with the built-in assumption that the claim is self-verifying. The main power of this tactic is that often the claim is not verifiable, forcing your opponent to prove a negative. It is a related tactic to a Red Herring, only designed to sound credible.
- Proof Surrogate
- This tactic is often paired with the Bare Assertion. Basically it is an appeal to authority without actually stating the authority. Often the identity of the person is only alluded to, and this is designed to ensure that the cited source is impossible to verify. Often proof surrogate users will cite a profession or title and leave it at that.