Common fallacies
Recognise the six reasoning mistakes you meet most often, in other people and in yourself, and answer them without turning a conversation into a contest.
Fallacy
A mistake in reasoning that makes an argument look stronger than it is. The ones in this lesson are 'informal' fallacies: the problem isn't the shape of the argument, as with affirming the consequent last lesson, but what it relies on, such as a distraction, a distortion or a missing option.
A fallacy is about the reasoning, not the conclusion. Someone can reach a true conclusion by a fallacious route.
FALLACY ONE
Ad hominem: attacking the person
Latin for 'to the person'. Instead of answering the argument, you attack the one making it: their age, job, past, looks or motives. The argument is still standing afterwards; you've just changed the subject.
'Why would I listen to you about saving money? You still live with your parents.' Whether she lives with her parents says nothing about whether her budgeting advice is good.
FALLACY TWO
Straw man: beating a scarecrow
You replace someone's actual view with a weaker or more extreme version, then knock that down. It's the opposite of the principle of charity from last lesson.
Parisa: 'I think we should eat out less this month.' Her husband: 'So you want us to never have any fun?' She didn't say that. He's arguing against a scarecrow he built.
Check yourself
At a team meeting Ali suggests testing the new app with ten real users before launch. Behrouz replies: 'Ali thinks we should delay the launch forever until it's perfect.' What has Behrouz done?
- An ad hominem: he attacked Ali personally
- A straw man: he swapped Ali's idea for a more extreme one
- Nothing wrong: he just summarised Ali's view
- A fair objection: any testing always causes long delays, so Ali is wrong
Show the answer
A straw man: he swapped Ali's idea for a more extreme one
Right. Ali proposed a small, specific test. Behrouz turned it into 'delay forever', which is much easier to reject.
FALLACY THREE
False dilemma: only two doors
The argument offers two options as if they were the only ones, when others exist. Pick the bad one and you're stuck with the other.
'Either you take this job in another city or you'll be stuck in a dead end forever.' What about applying elsewhere locally, asking for a new role, or retraining? Real choices usually have more than two doors.
FALLACY FOUR
Slippery slope: one step means the bottom
The argument claims a small first step will lead, through a chain of steps, to disaster, without showing why each step would really follow. Some slopes are real. The question is whether there's evidence for each link.
'If we let our son have a phone at twelve, he'll stop studying, fail his exams and ruin his future.' Each 'then' needs its own reason. Many twelve-year-olds with phones do fine.
Two more you'll meet everywhere
- Appeal to popularity: 'Everyone's buying it, so it must be good.' Lots of people can be wrong at once.
- Post hoc (from the Latin 'after this, therefore because of this'): 'I wore my lucky ring and passed, so the ring helped.' Coming after something isn't the same as being caused by it.
Check yourself
Match each line to its fallacy
Show the answer
- 'You're a student, what would you know about the economy?' → Ad hominem
- 'So you think rules don't matter at all?' → Straw man
- 'You're either with us or against us.' → False dilemma
- 'Skip one class and soon you'll drop out.' → Slippery slope
- 'The rain started after the new mayor arrived, so he brought it.' → Post hoc
Check yourself
A doctor tells Hoda to stop smoking. Hoda replies, 'But you smoke too!' Hoda has shown the advice is bad.
Show the answer
False
False. This is a kind of ad hominem, sometimes called 'you too' (tu quoque). The doctor may be a hypocrite, but whether smoking harms Hoda doesn't depend on the doctor's habits. The advice stands or falls on the evidence.
Answering a fallacy without a fight
- Don't shout its name
'That's a straw man!' usually ends the conversation. Describe the problem instead.
- Restate what you actually meant
'I'm not saying never eat out. I'm saying twice a month instead of weekly.'
- Ask about the missing link
'Why would one phone lead to failing school? What's the step in between?' or 'Is there a third option here?'
- Return to the real question
Bring it back to the point you were actually disagreeing about.
Check yourself
Fallacy, or a fair point?
- 'This supplement must work, it's the best-selling one in the country.'
- 'The report on this drug was paid for by the company that sells it, so let's look for an independent study too.'
- 'I drank ginger tea and my cold was gone in a week, so ginger tea cures colds.'
- 'Most climate scientists agree on this, so it's reasonable to take it seriously.'
- 'If you won't lend me money, you obviously don't care about me at all.'
- 'If we skip the car's service again, the small leak could turn into a costly engine repair.'
Show the answer
Fallacy: 'This supplement must work, it's the best-selling one in the country.', 'I drank ginger tea and my cold was gone in a week, so ginger tea cures colds.', 'If you won't lend me money, you obviously don't care about me at all.'
Fair point: 'The report on this drug was paid for by the company that sells it, so let's look for an independent study too.', 'Most climate scientists agree on this, so it's reasonable to take it seriously.', 'If we skip the car's service again, the small leak could turn into a costly engine repair.'
Lesson recap
- A fallacy is a mistake in reasoning that makes an argument look stronger than it is.
- Ad hominem attacks the person; a straw man attacks a distorted version of the view.
- A false dilemma hides other options; a slippery slope skips the evidence for each step.
- Popularity isn't proof, and 'after' isn't 'because of'.
- Name the problem gently, and remember a fallacy doesn't make the conclusion false.