Wishful Thinking

A reasoner who suggests that a claim is true, or false, merely because he or she strongly hopes it is, is committing the fallacy of wishful thinking. Wishing something is true is not a relevant reason for claiming that it is actually true.

Example:

There's got to be an error here in the history book. It says Thomas Jefferson had slaves. He was our best president, and a good president would never do such a thing. That would be awful.

Willed ignorance

I've got my mind made up, so don't confuse me with the facts. This is usually a case of the Traditional Wisdom Fallacy.

Example:

Of course she's made a mistake. We've always had meat and potatoes for dinner, and our ancestors have always had meat and potatoes for dinner, and so nobody knows what they're talking about when they start saying meat and potatoes are bad for us.

Weak Analogy

See False Analogy.

Untestability

See Unfalsifiability.

Unrepresentative Sample

If the means of collecting the sample from the population are likely to produce a sample that is unrepresentative of the population, then a generalization upon the sample data is an inference committing the fallacy of unrepresentative sample. A kind of hasty generalization. When some of the statistical evidence is expected to be relevant to the results but is hidden or overlooked, the fallacy is called suppressed evidence.

Example:

The two men in the matching green suits that I met at the Star Trek Convention in Las Vegas had a terrible fear of cats. I remember their saying they were from Delaware. I've never met anyone else from Delaware, so I suppose everyone there has a terrible fear of cats.

Most people's background information is sufficient to tell them that people at this sort of convention are unlikely to be representative, that is, typical members of society.

Large samples can be unrepresentative, too.
Example:

We've polled over 400,000 Southern Baptists and asked them whether the best religion in the world is Southern Baptist. We have over 99% agreement, which proves our point about which religion is best.

Getting a larger sample size does not overcome sampling bias.

Unfalsifiability

This error in explanation occurs when the explanation contains a claim that is not falsifiable, because there is no way to check on the claim. That is, there would be no way to show the claim to be false if it were false.

Example:

He lied because he's possessed by demons.

This could be the correct explanation of his lying, but there's no way to check on whether it's correct. You can check whether he's twitching and moaning, but this won't be evidence about whether a supernatural force is controlling his body. The claim that he's possessed can't be verified if it's true, and it can't be falsified if it's false. So, the claim is too odd to be relied upon for an explanation of his lying. Relying on the claim is an instance of fallacious reasoning.

Undistributed Middle

In syllogistic logic, failing to distribute the middle term over at least one of the other terms is the fallacy of undistributed middle. Also called the fallacy of maldistributed middle.

Example:

All collies are animals.
All dogs are animals.
Therefore, all collies are dogs.

The middle term ("animals") is in the predicate of both universal affirmative premises and therefore is undistributed. This formal fallacy has the logical form: All C are A. All D are A. Therefore, all C are D.

Two Wrongs Make a Right

When you defend your wrong action as being right because someone previously has acted wrongly, you commit the fallacy called "two wrongs make a right." This is a kind of ad hominem fallacy.

Example:

Oops, no paper this morning. Somebody in our apartment building probably stole my newspaper. So, that makes it OK for me to steal one from my neighbor's doormat while nobody else is out here in the hallway.

Tu Quoque

The fallacy of tu quoque is committed if we conclude that someone's argument not to perform some act must be faulty because the arguer himself or herself has performed it. Similarly, when we point out that the arguer doesn't practice what he preaches, we may be therefore suppose that there must be an error in the preaching, but we are reasoning fallaciously and creating a tu quoque. This is a kind of ad hominem fallacy.

Example:

You say I shouldn't become an alcoholic because it will hurt me and my family, yet you yourself are an alcoholic, so your argument can't be worth listening to.

Discovering that a speaker is a hypocrite is a reason to be suspicious of the speaker's reasoning, but it is not a sufficient reason to discount it.

Traditional Wisdom

If you say or imply that a practice must be OK today simply because it has been the apparently wise practice in the past, you commit the fallacy of traditional wisdom. Procedures that are being practiced and that have a tradition of being practiced might or might not be able to be given a good justification, but merely saying that they have been practiced in the past is not always good enough, in which case the fallacy is committed. Also called argumentum consensus gentium when the traditional wisdom is that of nations.

Example:

Of course we should buy IBM's computer whenever we need new computers. We have been buying IBM as far back as anyone can remember.

The "of course" is the problem. The traditional wisdom of IBM being the right buy is some reason to buy IBM next time, but it's not a good enough reason in a climate of changing products, so the "of course" indicates that the fallacy of traditional wisdom has occurred.

Tokenism

If you interpret a merely token gesture as an adequate substitute for the real thing, you've been taken in by tokenism.

Example:

How can you call our organization racist? After all, our receptionist is African American.

If you accept this line of reasoning, you have been taken in by tokenism.

Syllogistic

Syllogistic fallacies are kinds of invalid categorical syllogisms. This list contains the fallacy of undistributed middle and the fallacy of four terms, and a few others though there are a great many such formal fallacies.

Sweeping Generalization

See Fallacy of Accident.

Suppressed Evidence

Intentionally failing to use information suspected of being relevant and significant is committing the fallacy of suppressed evidence. This fallacy usually occurs when the information counts against one's own conclusion. Perhaps the arguer is not mentioning that experts have recently objected to one of his premises. The fallacy is a kind of fallacy of Selective Attention.

Example:

Buying the Cray Mac 11 computer for our company was the right thing to do. It meets our company's needs; it runs the programs we want it to run; it will be delivered quickly; and it costs much less than what we had budgeted.

This appears to be a good argument, but you'd change your assessment of the argument if you learned the speaker has intentionally suppressed the relevant evidence that the company's Cray Mac 11 was purchased from his brother-in-law at a 30 percent higher price than it could have been purchased elsewhere, and if you learned that a recent unbiased analysis of ten comparable computers placed the Cray Mac 11 near the bottom of the list. If the relevant information is not intentionally suppressed by rather inadvertently overlooked, the fallacy of suppressed evidence also is said to occur, although the fallacy's name is misleading in this case. The fallacy is also called the Fallacy of Incomplete Evidence and Cherry-Picking the Evidence. See also Slanting.

Superstitious Thinking

Reasoning deserves to be called superstitious if it is based on reasons that are well known to be unacceptable, usually due to unreasonable fear of the unknown, trust in magic, or an obviously false idea of what can cause what. A belief produced by superstitious reasoning is called a superstition. The fallacy is an instance of the False Cause Fallacy.

Example:

I never walk under ladders; it's bad luck.

It may be a good idea not to walk under ladders, but a proper reason to believe this is that workers on ladders occasionally drop things, and that ladders might have dripping wet paint that could damage your clothes. An improper reason for not walking under ladders is that it is bad luck to do so.

Subjectivist

The subjectivist fallacy occurs when it is mistakenly supposed that a good reason to reject a claim is that truth on the matter is relative to the person or group.

Example:

Justine has just given Jake her reasons for believing that the Devil is an imaginary evil person. Jake, not wanting to accept her conclusion, responds with, "That's perhaps true for you, but it's not true for me."

Style Over Substance

Unfortunately the style with which an argument is presented is sometimes taken as adding to the substance or strength of the argument.

Example:

You've just been told by the salesperson that the new Maytag is an excellent washing machine because it has a double washing cycle. If you were to notice that the salesperson smiled at you and was well dressed, this wouldn't add to the quality of the original argument, but unfortunately it does for those who are influenced by style over substance, as most of us are.

Straw Man

You commit the straw man fallacy whenever you attribute an easily refuted position to your opponent, one that the opponent wouldn't endorse, and then proceed to attack the easily refuted position believing you have undermined the opponent's actual position. If the misrepresentation is on purpose, then the straw man fallacy is caused by lying.

Example (a debate before the city council):

Opponent: Because of the killing and suffering of Indians that followed Columbus's discovery of America, the City of Berkeley should declare that Columbus Day will no longer be observed in our city.

Speaker: This is ridiculous, fellow members of the city council. It's not true that everybody who ever came to America from another country somehow oppressed the Indians. I say we should continue to observe Columbus Day, and vote down this resolution that will make the City of Berkeley the laughing stock of the nation.

The speaker has twisted what his opponent said; the opponent never said, nor even indirectly suggested, that everybody who ever came to America from another country somehow oppressed the Indians.

Stereotyping

Using stereotypes as if they are accurate generalizations for the whole group is an error in reasoning. Stereotypes are general beliefs we use to categorize people, objects, and events; but these beliefs are overstatements that shouldn't be taken literally. For example, consider the stereotype "She’s Mexican, so she’s going to be late." This conveys a mistaken impression of all Mexicans. On the other hand, even though most Mexicans are punctual, a German is more apt to be punctual than a Mexican, and this fact is said to be the "kernel of truth" in the stereotype. The danger in our using stereotypes is that speakers or listeners will not realize that even the best stereotypes are accurate only when taken probabilistically. As a consequence, the use of stereotypes can breed racism, sexism, and other forms of bigotry.

Example:

German people aren't good at dancing our sambas. She's German. So, she's not going to be any good at dancing our sambas.

This argument is deductively valid, but it's unsound because it rests on a false, stereotypical premise. The grain of truth in the stereotype is that the average German doesn't dance sambas as well as the average South American, but to overgeneralize and presume that ALL Germans are poor samba dancers compared to South Americans is a mistake called "stereotyping."

Stacking the Deck

See Suppressed Evidence and Slanting.