Summary: This resource covers using logic within writing—logical vocabulary, logical fallacies, and other types of logos-based reasoning.

Source: Purdue OWL Contributors: Ryan Weber, Allen Brizee

 

Fallacies are common errors in reasoning that will undermine the logic of your argument. Fallacies can be either illegitimate arguments or irrelevant points, and are often identified because they lack evidence that supports their claim. Avoid these common fallacies in your own arguments and watch for them in the arguments of others.

 

Slippery Slope: This is a conclusion based on the premise that if A happens, then eventually through a series of small steps, through B, C,…, X, Y, Z will happen, too, basically equating A and Z. So, if we don’t want Z to occur, A must not be allowed to occur either.

Example: If we ban Hummers because they are bad for the environment eventually the government will ban all cars, so we should not ban Hummers.

 

Hasty Generalization: This is a conclusion based on insufficient or biased evidence. In other words, you are rushing to a conclusion before you have all the relevant facts.

Example: Even though it’s only the first day, I can tell this is going to be a boring course.

 

Post hoc ergo propter hoc: This is a conclusion that assumes that if ‘A’ occurred after ‘B’ then ‘B’ must have caused ‘A.’

Example: I drank bottled water and now I am sick, so the water must have made me sick.

 

Genetic Fallacy: This conclusion is based on an argument that the origins of a person, idea, institute, or theory determine its character, nature, or worth.

Example: The Volkswagen Beetle is an evil car because it was originally designed by Hitler’s army.

 

Begging the Claim: The conclusion that the writer should prove is validated within the claim.

Example: Filthy and polluting coal should be banned.

 

Circular Argument: This restates the argument rather than actually proving it.

Example: George Bush is a good communicator because he speaks effectively.

 

Either/or: This is a conclusion that oversimplifies the argument by reducing it to only two sides or choices.

Example: We can either stop using cars or destroy the earth.

 

Ad hominem: This is an attack on the character of a person rather than his or her opinions or arguments.

Example: Greenpeace’s strategies aren’t effective because they are all dirty, lazy hippies.

 

Ad populum: This is an emotional appeal that speaks to positive (such as patriotism, religion, democracy) or negative (such as terrorism or fascism) concepts rather than the real issue at hand.

Example: If you were a true American you would support the rights of people to choose whatever vehicle they want.

 

Red Herring: This is a diversionary tactic that avoids the key issues, often by avoiding opposing arguments rather than addressing them.

Example: The level of mercury in seafood may be unsafe, but what will fishers do to support their families?

 

Straw Man: This move oversimplifies an opponent’s viewpoint and then attacks that hollow argument.

Example: People who don’t support the proposed state minimum wage increase hate the poor.

 

Moral Equivalence: This fallacy compares minor misdeeds with major atrocities.

Example: That parking attendant who gave me a ticket is as bad as Hitler.