How to Support an Argument and Avoid Logical Fallacies

What is an argument?

Every day we are presented with dozens of arguments that purport to be factual. Every day we must evaluate these statements and decide what we think about them—not only whether we agree with them or not, but also whether we think they are true. As you read this, you might take a minute to stop and think about how many such messages and statements you have already encountered today and how you thought about them.

Parts of an Argument

There are three basic parts to any argument, that is, three basic parts to any statement that is intended to persuade or prove something.

  1. Conclusion: This may or may not come at the end and is the author’s main idea
  2. Evidence: This is whatever the author or speaker uses to support the conclusion. (Below you will read about the various kinds of evidence, how to use them, and how to recognize when they are being misused.)
  3. Assumption (or Warrant): This is rarely stated explicitly in the argument, but is absolutely central. The assumption is what holds the argument together and determines how the author or speaker looks at the evidence and comes to the conclusion.

Evaluating Arguments by Identifying the Assumption

The following examples illustrate how you can identify an author’s assumption by looking at the other parts (evidence and conclusion) of their argument:

  • Evidence: Arbuthnot looks out his window and sees that it is snowing heavily.
  • Conclusion: Winter is a difficult and dangerous time of year.

What is Arbuthnot’s assumption in making this statement? Although we cannot know with absolute certainty, we can make a well-informed guess that his assumption has to do with how dangerous snow is. For instance, Arbuthnot may be afraid of driving in snow or afraid that he will fall while shoveling snow. Whether you agree with his conclusion or not, you can at least see his assumption at work. Here is another example:

  • Evidence: Montgomery looks out his window and sees that it is snowing heavily.
  • Conclusion: Winter is great! I look forward to it every year.

What is Montgomery’s assumption? Well, without more to go on we cannot say absolutely, but we can safely guess that it must be something that includes an enjoyment of snow. For instance, Montgomery might love downhill skiing, snowboarding, and snowmobile-riding or making snow angels. Do you see how this works?

So, in order to evaluate the worth of any argument, you must consider what the author’s assumption might be. In the examples, Montgomery and Arbuthnot have exactly the same evidence, and yet have come to very different conclusions. What makes the difference? They started from different assumptions. Thus, when you are evaluating an argument presented to you, ask yourself what the author’s assumption might be. Then, consider whether you think that is a sound assumption. For instance, here is an argument:

We are the best because we sell the most!

Have you ever heard this argument before? What is the assumption here? It is that quantity (of sales) equals quality (of product)? Do you think that is a sound assumption? Do you agree with it?

Tools for an Effective Argument

In addition to operating on a sound assumption, to argue convincingly the author must present evidence and analysis that support the conclusion.

Five common tools in an effective argument:
  1. Expert opinions
  2. Statistics
  3. Facts
  4. Examples
  5. Logic

Correct and Incorrect Use of Logic and Evidence

Evidence from Experts

The opinions of experts (also called authorities) can be invaluable. These are people who have some special knowledge on the subject, and their support lends believability to the author’s point. For instance, in a paper on why smoking is unhealthy, the U.S. Surgeon General and the chairs of either the American Lung Association or American Cancer Association would be strong authorities. Their authority comes from their professional experience. People become authorities for different reasons. They may have academic or professional training and experience, or they may also be people with extensive personal experience. Another authority on this topic might be a life-long smoker who now has extensive health problems.

However, using authorities may pose some dangers: The person may not be an authority in the right area. For example, “My lawyer told me about a great way to make my car run more fuel efficiently.” Advertising is notorious for using false authorities. For instance, Michael Jordan might be a good authority on the best basketball shoes, but does he really know more about underwear than anyone else? The classic example: “I may not be a doctor, but I play one on TV, so I know this is the best flu relief out there.”

Additionally, the topic may be hotly debated within the field—experts may disagree. When citing an authority, ask yourself not only if the person is truly an authority, but if they are in the area you are discussing. Also, note whether peers in the field generally accept what the authority says as true.

Evidence from Facts

Facts are proven to be true. For example, Helen Keller died on June 1, 1968. Support for this comes from many sources, one of which is her obituary in the The New York Times:

Facts are reliable sources of evidence. The problem comes in distinguishing fact from opinion. Opinions may look like facts, but they cannot be proven. For example, “The death penalty is wrong.” While one could gather evidence to support this opinion, one could also gather evidence against it. Ultimately, whether something is “wrong” or “right” is opinion.

Additionally, people may disguise opinion as fact by saying things like “The truth of the matter is . . .” or “It is a fact that . . .” but that does not mean what follows is fact. As a reader and writer, you need to evaluate what follows to determine if it is truly fact or opinion.

Evidence from Examples

Examples can be useful forms of evidence. In fact, in conversation, we often ask people to “give me an example.” For instance, your child comes home and says, “My teacher is not fair and does not like me.” You would most likely ask your child to give you some examples to support this claim. What could your child say back to you that would support the claim about the teacher?

Here are some possibilities: “They never call on me,” “They laugh when I give the wrong answer,” and so on. These examples clearly support your child’s claim, so they would be considered useful. What if, instead, your child said, “They wear the same outfit every day” or “They drink too much coffee”? Unless the teacher wears a shirt that says “I dislike Bobby” on it every day, you would say, “What does that have to do with how much they like you?” These examples do not support the claim. This example is an exaggeration, of course, but oftentimes people will use examples as evidence even though they really do not support their claims. You have to be on the alert for this as a writer and reader.

Using Logic

Authors use logic to build one known or proven fact upon another, leading the reader to agree that a certain point is true. For example, consider the case of Frank who has six cats. To prevent having even more cats, all Frank’s cats are female. Now consider his cat Zoe. What is Zoe’s sex? We know that all Frank’s cats are female, and we know Zoe belongs to Frank, so we can conclude that Zoe is a female. That is logic.

Logical arguments fall into two categories: induction and deduction. Deduction takes a generally known fact and uses it to argue for a more specific point. The example above of Frank and his cats is an example of deductive reasoning. Deduction is common. Induction, on the other hand, takes a specific case (or cases) and uses it to argue for a bigger generalization.

For example, it was hot today. It was hot yesterday, the day before, and the day before that. A logical conclusion is that it is summer. Induction is widely used in science. Scientists studying a particular species may notice that several individuals of that species exhibit a particular behavior.

They may then induce that all members of that species exhibit the behavior, even though they have not examined every individual member.

Induction is used less frequently because it can be faulty. For example, it was hot today. It was hot yesterday, the day before, and the day before that. What if it is not summer, but actually just a record-breaking October? Induction becomes more valid as the instances used to support the general conclusion become more specific.

Common Logical Fallacies

As seen above, even logical reasoning can result in incorrect conclusions. These are called logical fallacies. Several are common enough to have their own names and are described below.

Anecdotal Fallacy

Anecdotal fallacy, also called a hasty generalization or jumping to conclusions, is an inductive fallacy that occurs when one instance supports a general claim that is not true. For example, “I had a female boss once. She was demanding and unfair. Female bosses are the worst.” The speaker is using one example, the one female boss he or she had, to argue that all female bosses are horrid. Obviously—and your own personal experience may speak to this—they are not all bad.

Mistaking Time for Cause and Effect

Just because one thing happened prior to another does not mean the first caused the second, yet frequently people will argue just that. For instance, “The repair person was here this morning, and now my keys are missing. They must have stolen them.” The speaker could have just as easily misplaced them. Maybe their child grabbed them by accident. Just because A (the repair person at the house) happened before B (the loss of the keys) does not mean A caused B.

False Authority

When we trust authorities, we need to make sure they are authorities on the subject about which they are speaking. The speaker could tell you a great deal about acting, but not about your health.

Slippery Slope

Slippery-slope arguments are based on the idea that if one thing happens, then another thing will, then another, and another. Think of this as the “domino effect.” The problem is that while the first thing might lead to the next, there is no proof that any of the other things will happen. For example, “If I let my brother borrow this CD, then he will borrow my books, then he will want to borrow my clothes, and pretty soon, he will take over my bedroom!” (Children, among others, use this argument a lot.) While a brother allowed to borrow a CD might then ask to borrow a book, the guarantee that this will lead to room invasion is nonexistent. This logical fallacy should seem familiar—it is frequently used in political discussions on gun control and abortion.

Either-Or Dilemmas (A False Dichotomy)

“It is your choice—get a gym membership or be alone forever.” Are these really the only two options? Are there ways other than a gym membership to find that special someone? Of course there is. Either-or dilemmas, however, hide that fact by claiming only two options exist. Whenever you hear only two options, be suspicious. Other options likely exist.

Circular Reasoning

Circular reasoning, also called begging the question, happens when an assumption is used to prove the same assumption is true. In other words, the conclusion simply puts the assumption in other words. For example, “Jennifer Anniston is more popular than Courtney Cox-Arquette because more people like her.” Being popular and being liked are basically the same thing, so all this says is “She is more popular because she is more popular,” which is not saying much.

Ad Hominem

This literally means “against the person.” When a claim is rejected based simply on the person making it and not on the evidence the person has put forth, it is an ad hominem logical fallacy. The person, not the claim, is attacked. Why is this a logical fallacy? Because the claim the person is making is never attacked—only the person is. This personal attack is then used, falsely, to undermine the claim. Since the claim was never attacked, logically it has not been undermined, yet the attacker claims that it has. Here is an example:

  • Meg: I hope the NFL is able to keep its eligibility requirements. According to ESPN, there are 1200 agents registered with the NFL. Over half have no clients. If the eligibility requirements are dropped, these agents are going to be filling high school football stadiums, scouting for new prospects, and convincing a lot of kids to forgo college in hopes of an NFL career. The bottom line is most won’t make it, and what will they have? No education, no job, and no hopes.
  • Alan: You’re just a girl—what do you know about football?

For more examples of logical fallacies used in argument, refer to the Purdue Global Writing Center resource on Hasty Generalizations and Other Logical Fallacies.

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