Lecture: Fallacies of Relevance
Lecture script · about 15–20 minutes spoken · Week 7
Opening
Picture a student club's group chat. The treasurer posts a spring budget: 4 costs, 2 sources of money, and a total at the bottom. A minute later, a member replies, "We can ignore the treasurer's budget proposal; she failed a math class in high school." Three people react with a laughing face. Nobody checks the total.
That reply feels like a reason. It isn't one. A nursing student weighs a celebrity's health tip against a clinical guideline. A business major reads an ad that sells with fear. In each case, you ask whether a reason bears on the conclusion, so you can set aside what's loud.
Here's where we are. Week 1 taught you to find an argument. Week 2 gave you standard form and maps, with objections as arrows marked "against". Week 3 showed that an argument can be valid with a false premise. Weeks 4 and 5 tested deductive forms, and Week 6 watched words shift meaning.
We'll cover seven ideas. First, what a fallacy is, and how it differs from a false premise. Then the attacks on the person, the straw man, the appeals to fear, pity, and popularity, the red herring, and the appeal to authority. We'll finish with the classic source of the Latin names. After that, in Week 8, the course turns to the Midterm Review and Exam.
1. A broken link or a broken fact
Let's start with two friends arguing about a café. One says, "The café is closed for repairs this week, so we can't go tomorrow." If the café is really open, that argument fails. But notice where it fails. The reasoning is fine. The fact is wrong.
The other friend says, "Everyone I follow loves that café, so its food must be good." Even if that premise is true, it doesn't show the food is good. This time the facts can be fine while the reasoning fails.
Those are two different failures, and they have two different names. A fallacy is a flaw in the reasoning that connects an argument's premises to its conclusion, so that the premises fail to support the conclusion even when they are true. A false premise is a premise that is not true, a flaw in an argument's content rather than in its reasoning; an argument can have a false premise and still be valid.
This week's fallacies belong to one family. A fallacy of relevance is a fallacy whose premises, even if true, do not bear on whether the conclusion is true, such as an insult, a feeling, or a head count offered in place of reasons.
So here's the one test I want you to carry all week. Ask, "If this premise were true, would it bear on whether the conclusion is true?" If it wouldn't, the premise is beside the point, no matter how true or moving it is.
The study of fallacies begins with Aristotle's Sophistical Refutations, a work this course names without a date. The names you'll learn are a modern catalog. Think of it as a toolbox, not a rulebook.
Pause and try this. A hiker says, "All mushrooms in this forest are safe, and this one is from this forest, so it's safe." The first premise is false. Fallacy or false premise?
Run the relevance test on the reasoning before you look at the facts.
Here's the answer. It's a false premise, not a fallacy. If every mushroom in the forest were safe, this one would have to be safe, so the reasoning holds while a fact fails.
A fallacy breaks the link; a false premise breaks the facts.
2. Answer the reasons, not the person
Back to the group chat. If the budget adds up, it adds up whoever wrote it. That reply is the most common fallacy of relevance, and it comes in three versions.
The first is the attack on the person (ad hominem), the fallacy of rejecting a claim or argument because of a fact about the person who made it rather than because of the reasons offered. The treasurer passage is the plain case. A grade from years ago doesn't bear on whether this budget adds up.
The second is the you-too (tu quoque). It's the version of the attack on the person that dismisses advice or criticism because the adviser doesn't follow it, as if inconsistency made the advice unsound. Picture someone saying, "My doctor told me to cut back on soda, but she drinks soda herself, so I don't have to listen." Whether the adviser follows the advice doesn't bear on whether the advice is sound.
The third is the circumstantial attack on the person. It dismisses a claim because of the arguer's situation or interests, such as what they sell or stand to gain, instead of checking the reasons. Someone says the bike-shop owner supports bike lanes downtown only because he sells bikes, so his reasons don't count. His interest may be a reason to check his reasons. It doesn't show they fail.
Now, here's the neighbor that isn't a fallacy. Some facts about a person do matter, but only to one question: how much to trust what they tell you. A conflict of interest is a situation in which a person stands to gain or lose from what others believe, such as a paid spokesperson; it is a reason to weigh their testimony with care, not a refutation. And testimony is a report by a person that something is so, offered for others to rely on; its weight depends on the person's expertise, track record, and independence from the outcome.
Picture a paid spokesperson saying a blender is the quietest you can buy. Pointing out the payment is fair, but it doesn't show the blender is loud.
Pause and try this. A neighbor says, "The mechanic told me my brakes need new pads, but he's rude to everyone, so I'll skip it." What's the fallacy?
Ask whether rudeness bears on the brake pads.
Here's the answer. It's the attack on the person. Whether the mechanic is rude doesn't bear on whether the pads are worn, so the neighbor should check the pads instead.
Weigh the testimony, and answer the reasons.
3. Answer the real view
A city proposes closing one downtown parking lot one morning a week for a farmers' market. A resident writes in: "So its supporters want to ban cars from downtown." Then the resident argues, at length, that a car ban would hurt businesses. That might be a good argument. It answers a proposal nobody made.
Here's the term. A straw man is the fallacy of answering a weaker or distorted version of the opposing position instead of the position actually stated, so the reply knocks down a view nobody holds.
You already own the cure from Week 2. The principle of charity says to choose the reading that makes an argument as reasonable as possible. A straw man does the opposite. It picks the reading that's easiest to beat.
Let's try it on the running question, whether the campus library should stay open around the clock during finals week. You've had its standard form since Week 2: P1, P2, P3, which we supplied, and P4, with P4's cost of about 1,600 dollars in staff wages, leading to C. Here's the real objection. Overnight hours encourage all-night studying, and losing sleep can hurt exam performance, so the library would be helping students harm themselves. On the map, that objection is a box with an arrow marked "against" pointing at C. It's serious, and a reply has to meet it.
Now here's a straw man of the other side: "They want the library to be a dorm." No premise says students should live or sleep in the library. Knock that claim down, and every premise and the conclusion are still standing, because it attacks a box that isn't on the map.
Pause and try this. Someone answers the sleep objection by saying, "So you think students can't manage their own sleep." Is that answering the real objection?
Compare what the objection said with what the reply attacks.
Here's the answer. No, it's a straw man. The objection says overnight hours encourage sleep loss, not that students can't manage sleep, so the reply answers a claim nobody made.
Map the real view before you answer it.
4. What the feeling replaces
Picture a home-security ad with a dark window and a shadow. The voice-over tells you to install the app now, or your family will be the next victims of a break-in. It never says how the app works. It hands you a feeling and asks you to act on it.
Three fallacies of relevance work this way. The appeal to fear is the fallacy of offering fear of a bad outcome in place of evidence that the conclusion is true, as when an ad warns of disaster instead of showing that its product works. The appeal to pity (ad misericordiam) is the fallacy of offering sympathy for someone's situation or feelings in place of reasons that bear on whether the conclusion is true. The appeal to popularity (ad populum) is the fallacy of offering the number of people who believe something in place of reasons that bear on whether it is true.
Let's run the passages. The security app offers fear in place of evidence that the app works. A student says, "I should get an A in this course; I worked really hard, and I'll be crushed if I don't." Disappointment doesn't bear on whether the work met the standard for an A. And someone says, "Most people in the group chat think the new attendance rule is unfair, so it must be unfair." That's a head count offered in place of reasons.
Here's what I really want you to hear. Emotion isn't the problem. Suppose a forecast shows a strong storm heading for the coast, and a family feels afraid and leaves early. Their fear fits the evidence, and the evidence carries the argument. The fallacy is in letting a feeling stand in for reasons on a question the feeling can't answer.
Pause and try this. A friend says, "The storm forecast scares me, so I'm moving our beach picnic indoors." Is that an appeal to fear?
Ask whether the fear replaces the evidence or rides along with it.
Here's the answer. No, it isn't. The fear is a fitting response to a forecast, and the forecast is the reason, so emotion and evidence point the same way.
Ask what the feeling replaces.
5. Stay on the question
At a campus forum, a student asks the shuttle spokesperson a direct question: does the late-night shuttle run often enough? The spokesperson smiles and says the shuttles were recently repainted in the school colors. The room applauds. The question is still sitting there.
Here's the term. A red herring is the fallacy of changing the subject to a different issue and treating the new issue as if it settled the original one. The new paint is a different issue from how often the shuttle runs, so the passage commits the red herring.
Students mix this up with the straw man. A straw man stays on the issue but distorts the other side's view. A red herring leaves the issue entirely.
The cure is to write the original question down and hold it. When an answer arrives, check it against the question, not against how satisfying it sounds. Not every change of topic is a fallacy, either. "Good question; first, the shuttles were repainted, and second, they run every 20 minutes" changes the subject and then comes back. Check whether the original question ever gets answered.
Pause and try this. Asked whether a gym's showers get cleaned often enough, the manager talks about the new music system. Straw man or red herring?
Ask whether the reply distorts the question or leaves it.
Here's the answer. It's a red herring. The music system is a different issue, and the reply treats it as if it settled the question about the showers.
Write the question down, and hold the answer to it.
6. Check the expert
You take a pharmacist's word on a dosage and a mechanic's word on your brakes, and that's good reasoning, because you can't check everything yourself. The question is which appeals to experts deserve your trust.
An appeal to authority is an argument that a claim is true because an authority says so; it is good inductive evidence when the authority has relevant expertise, experts broadly agree, and no serious conflict of interest exists. An appeal to unqualified authority (ad verecundiam) is the fallacy of resting a claim on an authority when relevant expertise, expert agreement, or independence is missing, most often when the authority speaks outside their field.
That gives you three checks. Is this person an expert on this question? Do the experts in that field broadly agree? Does the authority stand to gain from your believing it?
Let's run them. "A famous actor says this herbal supplement cures colds, so it works." The first check fails at once. Acting isn't expertise in medicine, so the passage commits the appeal to unqualified authority. Now compare a national weather service forecasting a storm's path, which a club cites as a reason to move its outdoor event. The forecasters are experts on this exact question, they work from shared methods, and nobody profits from the club's choice. That appeal is relevant expertise on a question in its own field.
A good appeal to authority is inductive, the kind of argument Week 3 called strong. Forecasters can be wrong, but their forecast still makes the conclusion likely.
Pause and try this. A famous chef says a laptop is the most reliable one you can buy. Which of the three checks fails first?
Ask whether cooking is the right field for this question.
Here's the answer. The first check fails, relevant expertise. Cooking isn't expertise in laptop reliability, so the chef's word is an appeal to unqualified authority.
Right field, broad agreement, no stake.
7. Where the Latin names come from
You've heard a lot of Latin in this lecture: ad hominem, ad populum, ad verecundiam. These names have a history, and one book sits near its start.
Locke, in 1690, wrote An Essay Concerning Human Understanding. In Book IV, Chapter 17, he named the ad verecundiam and ad ignorantiam arguments and recorded ad hominem as a name already in use. He set all three against ad judicium, the argument from the evidence itself. So his chapter sorts arguments into four kinds: three that lean on something other than the evidence, and one that rests on it.
Here's the twist I want you to remember. For Locke, an ad hominem argument meant pressing someone with the consequences of their own principles or concessions: you already granted this, so you must accept that. That can be a fair move in a debate, because it holds people to what they've said. It's different from the modern attack on the person, which rejects a claim because of a fact about the speaker. The treasurer's old math grade is the modern kind.
Ad ignorantiam, the appeal to ignorance, waits until Week 13. And philosophers still argue about how to define several of these names, so this course keys only clear, neutral passages.
Pause and try this. Did Locke invent the name ad hominem?
Think about the difference between naming something and recording a name.
Here's the answer. No, he didn't. Locke named ad verecundiam and ad ignorantiam himself and recorded ad hominem as a name already in use, with his own older meaning.
Locke, 1690: two names his own, one already in use.
Closing
Let me pull it together. A fallacy is a flaw in the reasoning, and a false premise is a flaw in the facts, so you test the link and the facts separately. The attack on the person, the you-too, and the circumstantial attack answer the speaker instead of the reasons, while a conflict of interest bears on how much you trust testimony. A straw man answers a view nobody stated, so map the real objection first. Fear, pity, and popularity are fallacies when they replace reasons, not when emotion fits good reasons. A red herring changes the subject, so hold the answer to the question. Relevant, agreed expertise with no stake is good inductive evidence. And Locke, in 1690, gave us two of the Latin names and recorded a third.
Next in this module come the slide deck, the Readings and Resources page, the lecture tutorial, the practice set, and the quiz. Then, in Week 8, the Midterm Review and Exam sorts every test so far by the one distinction it asks for.
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