Reasoning can go wrong in any number of different ways. Sometimes an argument goes wrong in a totally unique and original way. But life is not always that exciting. Often arguments go wrong in a familiar, standard or common way. In that case, we say that the argument commits a fallacy.

    A fallacy is a common pattern of reasoning that is always or at least

    commonly bad.

(The qualification ‘at least commonly’ is there because sometimes an argument fitting the pattern is not in fact a bad argument; it just has the same form as others which (usually) are bad.)

Various Uses of the Word ‘Fallacy’

A fallacy is a kind or form of bad reasoning. However, the word ‘fallacy’ is also often used to describe a particular argument that happens to fit one of the standard fallacy forms. In such cases to say ‘that argument is a fallacy’ is shorthand for ‘that argument commits a fallacy’ or ‘that argument is an example of a fallacy’. People sometimes use the term as a colorful way of abusing an argument. In such cases, to say ‘that’s a fallacy!’ is just to say ‘that’s a bad argument.’ You should also be aware that the nature of fallacies is a subject of some dispute among logicians, and you might find somewhat different definitions of the term ‘fallacy’ if you look in other textbooks.

Names for Fallacies

Since fallacies are common patterns of (commonly) bad reasoning, and it is useful to be able to think and talk about bad reasoning, fallacies have been given names. Many of these names are quite arcane, usually because they are derived from Latin originals, e.g. petitio principii (begging the question) and argumentum ad hominem(personal attack).

An example of a fallacy: one common problem in reasoning is to use a key term one way in a reason, but another quite different way in the conclusion:

    Lesbians are not normal, because they are only a small minority of the population, and a minority isn’t normal.

If the conclusion of this argument is to be interesting at all, then the word ‘normal’ must mean something like OK or not deviant. But in the reason, the term ‘normal’ really just means in the majority. The argument tries to get you to accept a controversial conclusion on the basis of an uncontroversial premise, craftily changing the meaning of a crucial term along the way.

Since this kind of problem occurs quite often, it has been given a name. It is called the

fallacy of equivocation.

What are the Fallacies?

Dozens of fallacies have been identified and named. Good critical thinkers are familiar with most of the fallacies, and carry that knowledge in their heads, ready to apply it whenever they engage in reasoning. For people who have not yet memorized all the fallacies, there are some very good lists on the internet. One of the best is The Fallacy Files – http://www.fallacyfiles.org/.

Why Learn About Fallacies?

Knowledge of the fallacies is helpful in evaluating arguments. Instead of evaluating every argument laboriously ‘from scratch,’ you can often instantly recognize that an argument is committing a fallacy of a particular kind. Knowledge of fallacies will also help you avoid mistakes in your own reasoning. Finally, (we didn’t say this, but…) you can sometimes use fallacies to your rhetorical advantage. Many fallacies are common precisely because they are seductive, i.e., many people naïve in the art of argument will fall for them time and again. That is why politicians, for example, ‘commit’ so many fallacies.