October Study Group: Informal Logical Fallacies

Every month we also have a study/discussion group about an aspect of critical thinking, logic or argumentation. Our first one in October was about informal logical fallacies. Because there are a vast number of them, we only covered some of the more common and problematic ones.

Informal Logical Fallacies

I firmly believe that one of the main causes of strife in the world, both on a personal and global level, is a lack of critical thinking skills. We don’t rationally and critically evaluate what we hear or are taught, and often make decisions and form beliefs for irrational reasons without really criticizing or inquiring ourselves as to why. The purpose of this group is to learn, teach and practice evaluating both our own and other people’s arguments, whether in a formal debate setting or a casual one.

When we say “argument”, what do you think of? Do you think of two people verbally going at it, maybe ready to throw things? Well, some people think of it that way, but when our group talks about arguments, we are talking about something entirely different.

An argument is a rational and logical way of trying to uncover the truth behind an idea by asking critical questions, understanding how logic works and recognizing fallacies. It applies to both serious debates and everyday conversations and discussions. In terms of informal logic, the purpose isn’t to demonstrate a fact so we can’t “prove” it or say they are “true” or “false”, but rather it is an opinion or an evaluation, something that is accurate, or likely, or better than something else, so we can say they are “strong” or “weak”. So, for example, “cats are mammals so my pet cat is a mammal” is not an informal argument because it states a fact, it’s either true or it isn’t, but “you should stop smoking because smoking increases the chance of lung cancer” is. While the second statement (smoking increases the chance of lung cancer) is a statement of fact, the argument (you should stop smoking) is either strong (good) or weak (bad).

One aspect of good argumentation skills is recognizing informal logical fallacies. First let’s look at those words:


Does this mean “casual”? No, it doesn’t. Formal logic is logic relating to the form of the statements. For example, saying “Tomorrow I went to the beach” would be a formal grammatical error. So informal means logic that isn’t related to the form itself, but rather the thinking behind it.


This can basically be understood as reasoning or thinking based on a strict set of rules. Imagine you asked someone to play a sport with you and you explained all the rules to them, and they proceeded to break them in order to score a goal. Even if they scored the goal, would it still count? No, because you need to adhere to the rules in order to score a valid goal. It’s basically the same thing when it comes to arguments and logic; it doesn’t matter if your opinion is possibly “right”; if you get to it without using logic, you’ve broken the rules.


These are arguments that use bad or “false” reasoning.

So, informal logical fallacies are mistakes in the rules of arguing that have to do with the thinking behind the argument.

That might still be confusing, so I’d like to break it down a little more.

There are basically four types of informal logical fallacies:
1. Fallacies of ambiguity
2. Fallacies of presumption
3. Fallacies of weak inference
4. Fallacies of relevance


Arguments that commit fallacies of ambiguity manipulate language in misleading ways.
Fallacies of ambiguity appear to support their conclusions only due to their imprecise use of language.

The most common logical fallacy of ambiguity is probably equivocation.

This when people take a word to mean something different than the way it was originally used in the argument. In other words, the word is used to mean two different things.

a) The sign said “fine for parking here”, and since it was fine, I parked there.
b) The laws imply lawgivers. There are laws in nature. Therefore there must be a cosmic lawgiver.
c) Sure philosophy helps you argue better, but do we really need to encourage people to argue? There’s enough hostility in this world.

Another very common one is called a strawman argument, where the person misrepresents the argument and attacks that misrepresentation.

a) We can’t change the way we live to prevent global warming! What would we do without cars or electricity or gas?
b) We can’t reduce the military budget; what are we going to do without any military defense?
c) My teacher said I need to study grammar more, but I don’t have time to do that every day.
d) You’re an atheist, so you believe everything came from nothing.


Fallacies of presumption begin with an assumption which hasn’t been proven yet. Probably the most common example of this is the bald assertion, also called proof by assertion, which is simply stating a thing is true without any reasons.

a) It’s true!
b) That’s just the way it is.
c) It just IS!

When asking why, the person will usually just assert what the same thing, which is called a tautology (saying it’s true because it’s true), and is not an argument.

Another similar fallacy is the circular argument, also called begging the question. This is when the reasoning and conclusion both support each other!

a) I know he’s honest because he told me he’s honest.
b) this medicine cures colds because it’s cold medicine.
c) I know the bible is God’s word because the bible says so (the bible says it’s God’s word, so the bible is God’s word).

Another one is the false dilemma or false dichotomy. This is when the arguer offers only two options, but actually there are more. This is not always false; sometimes there are only actually two options, like “if Bob is not dead, then he’s alive” or “if she’s not a man, then she’s a woman”. But in many cases, there are sometimes many more options than what we are offered.

a) If we don’t reduce public spending, our economy will collapse.
b) If you don’t love your country, then leave.
c) If you’re not with us, then you’re against us.

The final common fallacy of presumption is called the argument from ignorance. We are all ignorant about some things to some degree, and the best thing we should do is learn until we know more. Some people, however, think that if they can’t think of anything else, then what they CAN think of must be true. This is the argument from ignorance. It is actually similar to the false dilemma, in that its inherent argument is “I can only think of two options, so there CANNOT be at least one more possible option.”

a) No one can actually prove that God doesn’t exists; therefore God exists.
b) No one on the council objected to the idea that he proposed, so everyone must think it’s a great idea.
c) I don’t see how evolution could increase the complexity of an organism (therefore, evolution is not true).

Weak inference

Arguments require evidence or good reasons to support them. When evidence or reasoning is weak, this is a fallacy of weak inference or weak induction. Certain appeals require weak evidence to support them.

Appeal to authority: My dad said it’s true (therefore it is true).
Appeal to popularity: Everybody’s doing it (there you should too).
Lots of people smoke, (therefore smoking is good).

Careful with the appeal to authority, though. It is NOT a fallacy if the person actually is an authority on the subject or topic, is not biased about the subject, and is named specifically. If the authority is not named (“Authorities say…”) or is biased or not an authority on the topic at hand is it fallacious. A consensus among other experts is also important; one scientist who believes there is a God is not sufficient as evidence for a God, for example.

Another type of weak inference is a generalization, specifically either a hasty generalization or a sweeping generalization. A sweeping generalization is one that takes a very broad example and assumes it is definitely true in a specific case. A hasty generalization is the opposite: it takes a specific example – which is not enough evidence – and attributes it to a broad group.

Sweeping generalization: All the men I’ve ever met were liars, so I don’t trust Bob, a man I met yesterday.
Every American I’ve ever met was individualist, and you’re American, so that means you’re an individualist.
Hasty generalization: Rachel, a feminist, says all men are sexist pigs, so feminists hate men.
I read a story in the news about this congressman who had an affair. Why do politicians always cheat on their wives?


When engaging in argumentation, it is important to stick to the topic at hand, rather than wandering off on tangents. Arguments that commit fallacies of relevance rely on premises that aren’t relevant to the truth of the conclusion. Fallacies of relevance are attempts to prove a conclusion by offering considerations that simply don’t bear on its truth or attempts to distract from the issue at hand. Arguments of this kind focus not on the evidence for a view but on the character of the person advancing it.

A classic informal logical fallacy of relevance is called an ad hominem attack. This is when the argument or counter-argument is focused on the person rather than the argument itself. The ad hominem does not have to be a rude insult. Often it appears logical, but is not.

a) YOU would say that.
b) Of course you think affirmative action is good, you’re a liberal/democrat.
c) How can you tell me smoking is bad, you used to smoke!
d) Sarah Palin said it, so it must be stupid.

Often, arguments use appeals to consequences or emotions, rather than logic or reason, to try to convince others that they are right. An appeal to consequences is an argument that because of bad actual, potential or imagined consequences, the truth or accuracy of the argument is wrong. While it may be important, it is not relevant to the truth of the argument. This fallacy is sometimes called wishful thinking.

a) Men and women can’t be different because if they are, then people will become more sexist.
b) I don’t want my existence to end, so there must be an afterlife.
c) An objective morality must exist, otherwise we would be free to do whatever we want.

Other arguments appeal to emotion, but again, while our emotions are important, they can’t tell us whether an argument is true or not. Manipulating other people’s emotions is an effective way to win an argument, but an ineffective way at reaching the truth. While your conclusions may be true, your argument isn’t if it has to appeal to an emotion.

Appeal to envy: Rich people have more than enough money, they should be giving it to the poor!
Appeal to fear: If you don’t graduate from college, you’ll live in poverty your whole life.
Appeal to flattery: You’re smart, so you must know Obama is a good president.
Appeal to pity: Look at that sweet, young girl. How could you think she could kill anyone?
Appeal to pride: You’re really smart, that’s why you agree with us.
Appeal to ridicule: You actually think there are UFOs? How silly.
Appeal to spite/hatred: You should cheat on him with his best friend because he’s a liar and an asshole.

A final one is a red herring. This is a very common fallacy that can be very hard to spot. It’s when introduces a totally different argument than the one you are talking about. While sometimes people will do it on purpose in order to distract you from your argument, often people will not even realize they are doing it, which is why it’s important to recognize it.

A) Person A: Murder is a criminal act, and should be punished. Person B: Yeah but lots of murderers have mental issues, and we really need to address that.
B) Person A: How can you eat all that junk food you know is bad for you? Person B: What should I do, let it go to waste? Think of all the poor people who can’t even eat anything.
C) Person A: I think the democrat’s presidential candidate has some bad economic policy ideas. Person B: Oh yeah? Well the republican’s candidate was accused of sexual harassment!
D) Person A: I think the presidents should have let those banks fail. Person B: But in tough times, we need to support our president.

Ok, now can you figure out the fallacy?

1. Most people like the TV show “Friends”, so it’s a good TV show.
2. The philosophy class I took was hard, so philosophy must be hard.
3. Bono from U2 talked about how bad capitalism is, so I realized it’s a bad system.
4. You can’t punish her for that, she has children!
5. Nobody has been able to prove that we don’t have psychic powers, so I think we do.
6. You’re a feminist, and feminists just want to get revenge on men, so why should I listen to your opinions on sexual harassment laws?
7. Giving your money to help out those in need is right, so we have a right to tax the wealthy.
8. You can’t say ask a woman her age, it’s just wrong!
9. He’s a good communicator because he speaks well.
10. You’re American, so you don’t understand Japanese culture, so your arguments about Japan don’t count.
11. Mining might be bad for the environment, but what about all the miners’ jobs?
12. If I don’t smoke, I’ll get fat, so I have to smoke.
13. If you care about the starving kids in Africa you will make a donation.
14. Everybody I’ve talked to doesn’t understand, so you won’t understand.
15. Evolution can’t be true, that means we’re just animals.


1. Appeal to popularity
2. Hasty generalization
3. Appeal to authority
4. Appeal to pity
5. Argument from ignorance
6. Straw man
7. Equivocation
8. Bald assertion
9. Circular argument
10. Ad Hominem
11. Red Herring
12. False dilemma
13. Appeal to pity
14. Sweeping generalization
15. Appeal to consequences

For more information, check out these websites:


Logical Fallacies and Constructing a Logical Argument
Informal Logic
More Informal Logic

Critical Thinking

What is Critical Thinking?


General Fallacies
Informal Fallacies Wiki
 Fun Site on Fallacies
What are Informal Fallacies?


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