Logical fallacies

You have to be careful with fallacies. The reason things are fallacies is because they're an invalid form of what's usually a valid argument. So it's not always clear whether something is a fallacy or not. In addition, some fallacies are only fallacies given certain other assumptions (see the naturalistic fallacy below).

Pure logic fallacies

post hoc ergo prompter hoc (after the thing therefore because of the thing): cause and effect fallacy

naturalistic fallacy: argues from nature that what exists is what should exist. Or, in other words, affirms that what is "natural" is what is "good".

Begging the question: assuming what you're trying to prove

Composition: Incorrectly arguing that something that applies to a part applies to the whole.

Division: Incorrectly arguing that something that applies to the whole applies to a part.

False dilemma (or "bifurcation"): limiting choices incorrectly when they shouldn't be limited. "You're either for A or against A". That may be valid, but maybe the issue is more complex and you can be for (or against) part of A but not all of A. This may, of course, be used as a social fallacy ("You're either with us or against us").

Non-sequiter: Your conclusion doesn't follow from your premises. Importantly, your premises may be true but be unrelated to the conclusion you draw from your argument.

"Social" fallacies

Appeal to emotion: appeals incorrectly to emotion rather than reason

Argument from outrage: argues against something without offering arguments besides saying that the thing would be unacceptible, or outrageous, or "wrong", or "silly", and so on.

Argument from popularity: argues for the truth of something based on the number of people who believe/do it. "A billion Chinese can't be wrong!"

Ad hominem ("against the man"): An argument against the person you're arguing with, not against his argument. Calling a person "stupid", for instance. Calling an argument "stupid" might also apply here, but that's probably something else like an "argument from assertion" or "argument from outrage". Either way, if someone uses emotionally-laden derogatory words during an argument, it's probably an ad hominem attack (ad hominem is kind of a catch-all).

Straw man: Arguing against a weak version of an argument and asserting that your argument applies to the stronger version. Or, misrepresenting your opponent's argument.

Genetic fallacy: argues against an idea based on its source. "Vegetarianism must be bad... Hitler was a vegetarian!"

See False dilemma above.

Language fallacies

Equivocation: using the same word in two different senses in an argument

Maybe composition and division as well.

Slippery slope: A slippery slope argument would be of the form "If they start registering guns, next they'll be rounding up our guns". To argue against gun registration because registering guns opens up the possibility of gun confiscation is not a slippery slope argument, because it doesn't argue that gun confiscation necessarily follows from registration.

Stephen's guide has a good page on the slippery slope argument. He helpfully describes the slippery slope argument as an invalid use of the "if, then" operator.

I've put the slippery slope under "Language fallacies" because during an argument there's often a way that you can make a slippery slope argument valid, but you have to be careful how you say it. Usually, one can argue that something undesirable is a logical consequence of something, rather than a necessary consequence. The slope might not wind up happening, but you can argue that the other person's premises allow it, or that it would simply be arbitrary, given the person's premises, if the slope didn't happen.

Of course, realize that often slippery slope arguments are good, in the sense that what they predict actually does happen. In this case, the person arguing probably understood something about human nature that his opponent(s) didn't. Unfortunately, subsequent events justifying the slope argument cannot be used at the time of the argument. Of course, if there are other cases of exactly what you're arguing for happening, it's certainly valid to cite those cases.

Evidential fallacies

Argument by assertion: merely asserting a point, or your position, without proving it. It's surprising how often people do it.

Appeal to (insufficient) authority: This is a particularly thorny fallacy, because often an appeal to an authority is justified, so long as the source is actually a legitimate authority. This fallacy refers specifically to the case where the authority you're using is not legitimate.

Hasty generalization: argues for a general point from too few or unrepresentative cases.

Argument from ignorance: Argues that something is true because you don't know it to be false. Or, vice versa.

Invalid appeal to common knowledge: "everyone knows that so and so is true". Or, "it's obvious that this is the case".

Other fallacies pages

http://datanation.com/fallacies/
http://www.infidels.org/news/atheism/logic.html
http://www.nizkor.org/features/fallacies/

Page last edited: March 31, 2005 (utc)