Avoiding Logical Fallacies

Put simply, a logical fallacy is a factual or reasoning error. If you base your argument on inaccurate evidence or use an inference that doesn't make sense, then you've committed a logical fallacy and your argument is said to be weak or invalid.

Before we start looking at specific examples of logical fallacies, there are a few terms you'll need to know.

A statement you assume to be true and that you use to build your argument. Think of a premise as the foundation of your argument.

Inference is the process by which you draw conclusions from your premises. Many of the logical fallacies we discuss below are a result of incorrect reasoning when trying to make an inference.

The conclusion is the necessary consequence of your premise and inferences: it's what you're trying to prove.

Fallacies of relevance occur when evidence that is unrelated to the argument is used to prove a conclusion. While the evidence being cited is true, it has no logical bearing on the argument.

Probably the best known fallacy of relevance is the ad hominem, or personal attack, where instead of going after the argument itself you attack the person making the argument. For example, if your teacher gives you a bad grade and instead of pointing out how good your paper was you just tell your teacher she's ugly, you've committed this fallacy.

When you appeal to the growing popularity of an idea to prove its truthfulness, you've committed the bandwagon fallacy. There are lots of reasons that large groups of people might believe an idea, so you can't necessarily assume that because lots of people believe it that it's true.

The fallacy of composition is assuming that because something is true of all the parts of a whole, that thing is true of the whole as well. The fallacy of division is the opposite: assuming that because something is true of the whole, it is true of the parts as well.

The gamblers fallacy occurs when you assume that because a statistically unlikely event has occurred, a balancing event is more likely to occur in the short term. This is very common in sports. For example, because a basketball player has missed ten free-throws in a row, the announcer will say that he's bound to make the next one. In fact, the player's previous free-throws have no impact on whether he'll make the next one.

This is another of the most common fallacies of relevance. An irrelevant appeal is an attempt to persuade the listener by relying on emotions to prove an argument. These include appeals to authority (it's true because the teacher said so), consequences (it's true because if you don't believe it bad things will happen), force (believe this or I'll hurt you), pity (my story is sad so it must be true), and tradition (it's true because we've always believed it).

The moralistic fallacy is assuming that the world works as it should, meaning because something ought to be true that it is. The naturalist fallacy is the opposite: assuming that because something is true then that's how it should be.

Just like the name suggests, a weak analogy is an analogy where the relationship between the two items being compared is too weak to support to the argument. For example, if I say "I'm like a teacher because I write on the chalk board, therefore I should be able to grade my own paper" the comparison of "me" to "teacher" is not strong enough to support the conclusion.

Fallacies of ambiguity happen when words are used incorrectly or imprecisely so that they seem to support an argument when in fact they don't.

An equivocation is when the same word is used to mean two different things within the same argument. For example, in the argument "All banks are besides river, therefore, the place where I deposit my money is next to a river," the equivocation fallacy occurs because the word "bank" has two different meanings. This type of fallacy often comes from the use of words with vague or multiple meanings like religion, belief, or truth, and it can be avoided by clearly defining your terms at the beginning of any argument.

A straw man argument is when you intentionally misrepresent your opponent's argument in order to make it seem weak. Then, when you prove the argument false, the opponent's original argument is "proved" wrong. In the following exchange,

When an argument relies on a vague or incorrect premise, that's known as a fallacy of presumption.

Affirming the consequent is an argument that takes the from "If A then B, and B, so A." For example, if my boyfriend was cheating on me he'd stay at work late. My boyfriend stayed at work late, so he's cheating on me." Obviously, there are any number of reasons my boyfriend might have stayed late at work, which is why you can't assume an "if" statement is true if you only have the consequence.

When you say something is true only because you can't prove it's false, that's called arguing from ignorance. The classic example of this is saying "Because nobody can disprove the existence of God, then he must exist."

In casual conversation, begging the question is commonly used to mean "raises the question," for example "her absence begs the question, where has she been all night?" But in the vocabulary of logical arguments, begging the question has a different definition: it means to include your conclusion as a premise. Basically, if you need to assume that your conclusion is true in order to prove it, then that's a logical fallacy (that's why begging the question is also called circular reasoning). For example, "my friend said she would never lie to me, and my friend never lies, so therefore my friend didn't lie to me."

A complex question fallacy is a question that rests on a improvable assumption and to which any answer confirms that assumption. For instance, if you ask someone "Are you ready to admit you're wrong?" whether they answer yes or no, they have to admit they were wrong in the first place.

Assuming that because two things happen together that one causes the other is known as a cum hoc fallacy. In common language, this fallacy is often stated as "correlation does not equal causation."

A false dilemma is when a choice is presented between two options when there are other options available. For example, "if my teacher gave me a bad grade, it's either because she doesn't like me or because she read my paper wrong."

When counter-examples are dismissed as irrelevant just because they disprove an argument, it's called the no true Scotsman fallacy:

The post hoc fallacy is similar to the cum hoc fallacy: it assumes that because one thing happens after another, then the first necessarily caused the second.

A slippery slope argument assumes that one thing will necessarily lead to another. They're very common in political argument; for example, "if people are legally allowed to use marijuana, then soon they'll start using more dangerous drugs."

A sweeping generalization (also called a hasty generalization) is a conclusion drawn from a single piece of evidence, for instance, "my dog peed on the floor, therefore no dogs can be house trained."

When you assume that just because someone else did something then it must be right. It's a favorite of children who, went caught doing something wrong, say "well my sister did it too!"