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A fallacy is an error in logic or reasoning that leads to an argument supported by illogical or misleading premises. In some cases, particularly in advertising and in informal arguments, fallacies are intentionally used in order to sway the opinions of others. Close analysis of an argument that relies on a fallacy, however, always reveals that the conclusion of the argument cannot be drawn from the premises. Fallacies come in the form of sweeping generalizations, appeals to emotion or authority, assumptions of causality, and a variety of other statements based outside of logic. While some fallacies are made intentionally, it is easy to make accidental logical errors, so it is important to analyze one's own arguments at least as rigorously as one examines the arguments of others.
There are many different types of fallacies that are classified based on the types of logical errors that they involve. Some types involve generalizations — one commits a logical fallacy by drawing a specific conclusion from an untrue generalization or by inferring a general rule from one specific case. Another type of fallacy involves setting up a false choice by stating that there are only a small number of possible solutions to a given issue when there are, in fact, many more. Many other forms of fallacy also exist, almost all of which involve reaching a conclusion based on illogical premises or taking a conclusion for granted without reason to do so.
A verbal fallacy is a type of fallacious statement that is based on misuse of words. Such fallacies are often based on ambiguous words and phrases. Using an ambiguous word in two different ways in the same argument, for instance, is known as "equivocation" and is a common verbal fallacy. Other verbal fallacies simply involve using a great many words and phrases to make an argument that sounds good but is difficult to unravel.
Some logical fallacies appear logically correct and are effective simply because they are difficult to distinguish from logical arguments without rigorous examination. Others appeal to one's emotions and biases or to some authority figure. One may choose not to argue a point that is based on the word of a famous professor, for example, simply because by protesting the point, one is also opposing a well-respected figure. This fallacy and others that rely on emotion and coercion do not necessarily appear logical, but they appeal to a person's emotions or sense of ethics regardless of logical considerations.
@Kat919 - Oh, those make me so mad! So many articles just don't make it clear that one thing doesn't necessarily cause another just because you find them together.
Another post hoc fallacy example is a recent study showing that kids who spent a lot of time on Facebook and other social media are more likely to drink and engaged in other risky behaviors. The reporting made it sound like Facebook *causes* drinking.
How about this: kids with a lot of friends, or who are very concerned with their public image, are more likely to drink, and also more likely to go on Facebook?
Obviously, the study doesn't show that Facebook use *doesn't* cause drinking. But it's important to remember the correlation is not causation.
The press loves to the post hoc fallacy. In full, it's "post hoc, ergo propter hoc," which in Latin means "After this, therefore because of this."
The idea with the post hoc fallacy is that we tend to look at something that comes after something else and think that the first thing *caused* it. There are a lot of stories about correlations, and the press reports it as if correlation and causation are the same.
For instance, you see studies that kids who spend time in day care are more aggressive, have more behavior problems, etc. Well, it doesn't prove that day care *caused* those behavior problems. Could be that Mom went back to work because the kid was a handful! Prime example of the post hoc fallacy.
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