Classical Rhetoric 101: Logical Fallacies

by Brett & Kate McKay on May 26, 2011 · 47 comments

in Blog

I hope you’ve enjoyed our series on classical rhetoric. Today, for our final entry in the series, we will be discussing logical fallacies. This is a pretty important topic. Much of our debating has moved out of the public square and onto the internet. And if you spend anytime reading those online debates, you’ve probably seen how debased from the principles of classical rhetoric many of them are. Commenter X sets forth a fiery opinion on an article. Commenter Y responds by calling Commenter X a Giant Poopie Head for holding said opinion. And Commenter Z joins in with a tirade on a point that is not even argued in the piece.

For true civil and effective debate to take place, citizens must understand not only how to argue, but how not to argue as well. Every man should know how to avoid the pitfalls and traps of faulty arguing and how to recognize fallacies in the rhetoric of others as well.

What Are Fallacies?

According to Aristotle in his treatise, The Art of Rhetoric, a speaker or writer has three ways to persuade his audience: ethos (appeal to the speaker’s character), pathos (appeal to emotion), and logos (appeal to logic). Aristotle believed that out of the three means of persuasion, logos was superior and that ideally all arguments should be won or lost on reason alone.

The problem with using logos as your sole means of persuasion is that it’s fraught with many opportunities for you to mess up and make errors in reasoning. These errors are called logical fallacies.

Just as there is formal and informal logic, there are formal and informal fallacies. Below we provide a quick intro to formal and informal fallacies and give examples from both.

Formal Logical Fallacies

Aristotle was a big fan of formal syllogisms. In fact, he wrote a whole treatise on them. There’s a reason why he liked them so much. Syllogisms are a powerful rhetorical tool. It’s hard to manipulate and argue against a formally laid out, sound syllogism.

A formal fallacy in syllogisms occurs whenever the structure of the argument itself is flawed and renders the argument invalid. The premises and conclusion of the argument might be true, but the argument can still be fallacious because the conclusion doesn’t follow from the premises. In my post about the three means of persuasion, I gave an example of an invalid syllogism:

All men are mortal.
Socrates is mortal.
Therefore, Socrates is a man.

At first blush, it looks like a decent argument. But read it carefully. Just because Socrates is mortal, doesn’t necessarily mean he’s a man. He could be a squirrel for all we know. This right here is an example of the fallacy of the undistributed middle. Let’s take a look at a couple of other formal syllogistic fallacies.

Affirmative conclusion from a negative premise:

No cats are dogs.
No dogs can purr.
Therefore all cats can purr.

Just because there aren’t any dogs that can purr doesn’t necessarily mean all cats can purr.

Negative conclusion from affirmative premises:

It’s impossible for a negative conclusion to be reached with affirmative premises.

All gods are immortal.
All immortals have beards.
Therefore, no gods have beards.

Informal Logical Fallacies

Informal fallacies are arguments that are fallacious for reasons other than a flaw in the structure of the argument. You’re probably familiar with a few informal fallacies already: red herrings, slippery slopes, etc. Below we list several of the most used informal fallacies to look out for when taking part in a debate.

  • Red herring-an attempt to change the subject to divert attention from the original issue. You can see countless examples of this when you watch presidential candidates debate. Example: “Yes, I would absolutely make the wars in Afghanistan and Iraq a priority. But with the unemployment rate as high as it is, we really need to concentrate on domestic issues and creating jobs, and under my plan….”
  • ad hominem- attacking the person instead of the argument. The goal is to discredit the argument by discrediting the person advocating the argument. Ad hominem attacks are popular in online discussions, especially when tempers flare. “Well, you’re wrong because you’re clearly an idiot!” That sort of blatant insult is easy to spot. Harder to detect are arguments that go something like, “Well, I don’t believe what Politician X has to say about the tax plan because he has said some absolutely crazy things in the past.” It may be true that Politician X has proven himself to be a nut job on a variety of issues, and this may affect his ethos, but it does not logically disprove what he has to say about the tax plan. He might be wrong on everything but this one issue.
  • Argumentum ad populum- concluding an argument is true simply because lots of people think it’s true. We see this on commercials all the time: “9 out of 10 doctors recommend Acme Brand Toothpaste,” or “3 million Brand X Customers Can’t be Wrong! Buy Brand X Today.”
  • Appeal to authority- concluding an argument is true because a person holding authority asserts it is true. “Doctor Who is an expert in quantum physics. If he says time travel is possible, then it must be true!”
  • Appeal to emotion- instead of appealing to reason, the arguer uses emotions such as fear, pity, and flattery to persuade the listener that what he says is true. Wartime propaganda posters are a good example of an appeal to emotion:
  • Appeal to motive- a conclusion is dismissed by simply calling into question the motive of the person or group proposing the conclusion. You’ll often see political organizations use this tactic. “The conclusion of Company X’s positive report on the safety of natural gas fracking can’t be true because they funded the research and have an interest in ensuring there is a positive report.” Sure, Company X may have an interest in getting a positive result for natural gas fracking, but just because they have that motive doesn’t mean the conclusion they reached is necessarily false. Suspect, yes, but not false.
  • Appeal to tradition- concluding an argument is true because it has long been held to be true.
  • Argument from silence- reaching a conclusion based on the silence or lack of contrary evidence. Example: “Aliens must not exist because we haven’t made contact with them.”
  • Reductio ad Hitlerum- comparing an opponent or their argument to Hitler or Nazism in an attempt to associate a position with one that is universally reviled. People seem to use this one a lot on the web. Example: “You know who else was a vegetarian? Hitler. Therefore, vegetarianism is bad.”

  • Strawman- an argument based on an misrepresentation of an opponent’s position. It’s called a strawman because the person sets up a false point (the strawman) that the original arguer never made and expends all his energy attacking it, instead of the actual premises of the original argument. Example: “Senator Smith wants to cut funding for the new Air Force fighter jet because he says it’s wasteful spending. I disagree with the Senator’s stance. Why does Senator Smith want to leave our country defenseless?” Instead of debating whether the jets are actually government waste, the arguer ignores that point and instead substitutes a misrepresented version of the senator’s position, i.e. the senator wants to leave our country defenseless.
  • Appeal to hypocrisy- an argument that a certain position should be disregarded or is wrong, based on the fact that the proposer of the position fails to act in accordance with that position. Example: “Your point that entitlement programs should be eliminated is moot based on the fact that you’ve received Pell Grants and used food stamps while in college.” Sure, it’s hard to take someone seriously when they’re simultaneously using government programs and arguing for their elimination, but just because a guy doesn’t practice what he preaches, doesn’t automatically make what he’s preaching false. Instead, the debate should be focused on the pros and cons of government programs themselves.

Slippery Slope

Slippery slopes occur when a person asserts that a relatively small step will lead to a chain of events that result in a drastic change. Example: “If we legalize same sex marriage, what will stop us from legalizing marriage between humans and robots? Or humans and animals?”

Cherry Picking

Fallacy that occurs when a person only uses data that confirms a particular position, while ignoring a significant portion of related cases that contradict that position. For example, a person might argue that a vegan diet prevents cancer while ignoring cases of cultures that eat only meat and have very low cancer rates.

Begging the Question

Fallacy that occurs when the conclusion of an argument is assumed in one of the premises. It’s also often called circular reasoning. If one’s premises entail one’s conclusion, and one’s premises are questionable, one is said to beg the question.

Here’s one of my favorite webcomics, Dinosaur Comics explaining begging the question:

Post hoc ergo propter hoc

Latin for “after this, therefore because of this.” A fallacy that occurs when someone reaches a conclusion of causation because an event followed another event. Example: “It started to rain after my ice cream cone fell on the ground. Therefore, my ice cream falling on the ground caused it to rain.”

False Dilemma

A fallacy that occurs when two conclusions are held to be the only possible options, when in fact there are other options. Example: Senator A: “We either have to cut education spending or else we’ll have a huge deficit this fiscal year.” Senator B: “Hmmm…there are other options. You could raise taxes or even cut spending in other programs and agencies.”

This list is by no means exhaustive. There are dozens more informal fallacies. If you’re interested in finding out more about informal fallacies, check out the following resources:

I’d love to read what informal fallacies are your favorites to point out. Share them with us in the comments.

Using Informal Fallacies to Persuade

Reading through that list of informal fallacies, you likely stopped a few times, and thought, “Wait, but isn’t that a persuasive argument? Shouldn’t we appeal to experts, to tradition? Isn’t a slippery slope possible? Shouldn’t the character of the messenger have something to do with whether their message is believable?”

One must remember that that while sometimes they can be one and the same, there can be a difference between an argument that is logical and one that is simply persuasive.

And sometimes it’s okay to use the latter.

What the what? Only a cad would purposely use informal fallacies in an argument, right? Well, yes and no. It’s important to remember that rhetoric is fundamentally about persuasion, and not only about crafting arguments that are perfectly logical. If we weren’t allowed to use informal fallacies in our rhetoric than two of the three means of persuasion would be off limits–ethos (appeal to the speaker’s character) and pathos (appeal to emotions). Both are informal logical fallacies.

Politicians and advertisers understand that human beings are persuaded more by emotion than by reason. That’s why you see politicians and advertisers use informal fallacies all the time.

It does create an ethical dilemma for a rhetorician. Should one eschew appeals to emotion or character because they’re not strictly logical? While using only logic and sound reasoning may seem like taking the high road, a rhetorician risks being ignored and never getting an important message to the masses. Some of the greatest speeches in history were based on both logic and emotion. Going back to the example of propaganda posters during WWII…the government needed to fund the war effort quickly and effectively; simply appealing to logic wouldn’t have accomplished what was necessary. Instead, appealing to Americans’ sense of duty, patriotism, and worry for their loved ones motivated citizens to action. Used for honorable purposes, appeals to ethos and pathos can move people to do great things. And of course in the wrong hands, they can persuade people to do evil. That’s why it’s important to have an informed and intelligent citizenry that is able to evaluate the claims and appeals made by leaders and pundits, allowing themselves to be caught up in emotion when the cause is sound, and cutting through the illusion when it is not.

Everyone is going to have a different answer as to whether or not using informal fallacies is justified. I think the key is to find a balance. Use appeals to emotion or character, but always have some actual facts and sound reasoning to back up those appeals.


Well, that does it for this series on classical rhetoric. We plan on putting the posts together into an ebook for those who are interested in having all the posts in one place. Although such projects always seem to take us forever to finish, since they’re often placed on a distant burner on the large AoM oven.

I’ll leave you with a list of resources that were helpful to me in researching this series and can help you continue your study of classical rhetoric.

Classical Rhetoric 101 Series 
An Introduction
A Brief History
The Three Means of Persuasion
The Five Canons of Rhetoric – Invention
The Five Canons of Rhetoric – Arrangement
The Five Canons of Rhetoric – Style
The Five Canons of Rhetoric – Memory
The Five Canons of Rhetoric – Delivery
Logical Fallacies
Bonus! 35 Greatest Speeches in History

{ 47 comments… read them below or add one }

1 Steve Lewis May 26, 2011 at 10:54 pm

I teach college communication courses, and I often will use the Simpsons episode “Much Apu About Nothing” to illustrate a large number of logical fallacies. The episode concerns a vote on a proposition that would send all illegal immigrants in Springfield back to their home countries. But the episode is written upon the foundation of dozens of logical fallacies. Lisa Simpson even points out one to Homer: Homer notes that the newly-instated bear patrol in Springfield has kept all of the bears from invading people’s homes. Lisa points out the lack of logic in his statement, and suggests that the rock she is holding might just as easily be keeping tigers away. I show the entire episode in my class, and we record and discuss the various logical fallacies that students are able to identify. It’s great fun!

2 James May 27, 2011 at 1:12 am

My favorite is probably the complex question (plurium interrogationum). For example, when somebody asks “why do you hate me?” when the assumption of hate was made without any real basis. Other examples could be as ridiculous as asking “what color is Tuesday?”

3 Oolon Colluphid May 27, 2011 at 1:43 am

I think there’s a fairly simple way of telling when using ethos and pathos is acceptable. If what you’re trying to say is supported by reason, but ethos or pathos are more able to convince people, then it’s acceptable. In this case, if practical, you should make the logical arguments available for people who aren’t affected by ethos or pathos. For example, a politician might appeal to the desires of his constituents in a speech, but on his website or in his handouts he should then include the data and logical arguments that are the actual reasons it’s a good idea.

If a conclusion is not supportable by reason, then it is dishonest and wrong to argue for it by any means.

4 Andrew May 27, 2011 at 2:20 am

I took a philosophy class seven years ago in university called “The Art of Thinking”. We used a book called, “A Practical Study of Argument (Fifth Edition)” by Trudy Govier. It discusses all the fallacies in this post and others, syllogisms and more. I still have it.

5 Cpt. Lars May 27, 2011 at 6:29 am

I love using a bit of reductio ad absurdum when having a heated discussion. I find it often makes a person’s mind stop and contemplate the nonsense I’ve spouted just long enough for me to seamlessly slip another fallacy straight in there and totally derail their train of thought!

It’s inherently caddish behaviour, but I do enjoy running mental circles around people, logical or not.

6 JG May 27, 2011 at 9:14 am

Does anyone know the name of this rhetorical technique:

Candidate 1: I won’t spend time talking about how Candidate 2 flip-flops on important economic issues; I won’t mention the fact that Candidate 2 was an alcoholic; I won’t waste your time addressing the fact that Candidate 2 is anti-Semitic. No, that would just be a waste of time and not very wise debating.

….Candidate 1 spent time talking about his opponents faults while saying that he wouldn’t.

Is that double-speak?



7 Dave May 27, 2011 at 9:38 am

Great series! Unfortunately, not all the articles in it have been tagged with “rhetoric” so they don’t all appear together here:

As a result, some of them are hard to find. For example, the introduction (which itself is not tagged) mentioned that the series would cover the Three Genres of Rhetoric (deliberative, forensic, and epideictic), but I can’t find the articles about them. Did I miss them?

8 dave May 27, 2011 at 9:47 am

That’s called paralipsis, it’s a subset of apophasis:

9 Dan May 27, 2011 at 10:54 am

One of my faves (and by fave I mean over-heard and well-hated) is the No True Scotsman fallacy. If a person states “we don’t engage in behaviour X,” and you show proof that the group does, in fact, regularly engage in such behaviour, the response is often, “well, no REAL one of us would do so,” or “they’re not really (or never were) true members of our group.” Warm up the bagpipes.

10 Andrew#2 May 27, 2011 at 11:36 am

Great article!

One of the more common logical fallacies I like to call “Person You’re Arguing With Is An Idiot”.

The true Idiot is not one who disagrees with you. In fact, they are the uncle, neighbour, or professor who start hopping around howling like a scorched monkey when you disagree.

The Idiot violates every logical fallacy on their way to complete irrelevance, bouncing from wall to wall and occasionally back into said fire. The only one who really suffers is you.

Therefore, the real logical fallacy I am presenting is the very act of debating with an Idiot.

11 Steve May 27, 2011 at 1:56 pm

Schopenhaur’s “The Art of Controversy”, a guide to winning any argument by Sophisms. The middle section of the book is a list of 38 stratagems.

12 Elijah May 27, 2011 at 4:24 pm

I’m glad you put in that bit about persuasiveness at the end because almost every slippery slope argument I remember from 20 – 25 years ago has proven to be exactly correct.

13 ASR May 27, 2011 at 6:52 pm

Ah, the red herring. Bane of many law student’s existence.
Anyway, my favorite is Reductio ad absurdum. Although, I am not too sure it is a fallacious argument, but it certainly is one of my favorites to use. For those who don’t know it, you prove the assertion false by following it to its conclusion which results in an absurd result.

You could certainly use it falsely by creating a large chain of events that do not follow logically from the premise to your absurd result. Well, all this thought of logic keeps making me think of Venn diagrams, and I really don’t like those. Those make me think of long chains of logical statements reduced to math. And that just makes my brain hurt.

14 Chris May 27, 2011 at 7:22 pm

one of my favs, “Reductio ad Infant-itum”. Where an appeal to children is made for the supposedly simple truth of an idea. “Even a child knows X to be true.” Disregarding the fact that these same children eat their boogers.

15 Jonathan May 27, 2011 at 7:30 pm

Chris, excellent point. I even know some adults who eat their boogers.

Loved this series; pumped for the e-Book!

16 Jack May 27, 2011 at 8:07 pm

There’s a lot of ad hoc reasoning, especially in biology and psychology. Evolutionary psychology and physiology seem to be especially afflicted, which is a shame because they would be such interesting topics but the fields turn out to be full of BS due to ad hoc reasoning.

For example, just the other day there was an article here on AoM that said that men have more sweat glands than women, so they are better suited to running and other tasks that generate a lot of heat. But they also have much more body hair than women, so what’s the evolutionary point of that? If men had less hair than women, they’d just conclude that mens’ body hairlessness was an adaptation to keeping cool (and aerodynamic) while running.

I recently heard that men have better night vision than women, since men are hunters who hunt at night. More BS. Hunter-gatherer humans rarely hunt at night, and those who have been documented doing so are setting up nets with bells on them to let them know when a bat or other prey has been ensnared. Humans are primarily diurnal hunters. Furthermore, studies show that female humans tend to have more rods in their retinas than males, so in theory their night vision would be better – maybe for taking care of fussy babies at night or some other reason ad hocly pulled out of a researcher’s behind. But studies on practical night vision in males and females haven’t shown significant differences either way.

17 Sean May 27, 2011 at 9:09 pm

@ Jack:
“For example, just the other day there was an article here on AoM that said that men have more sweat glands than women, so they are better suited to running and other tasks that generate a lot of heat. But they also have much more body hair than women, so what’s the evolutionary point of that? If men had less hair than women, they’d just conclude that mens’ body hairlessness was an adaptation to keeping cool (and aerodynamic) while running.”
this pretty much explains it:

“Furthermore, studies show that female humans tend to have more rods in their retinas than males, so in theory their night vision would be better – maybe for taking care of fussy babies at night or some other reason ad hocly pulled out of a researcher’s behind. But studies on practical night vision in males and females haven’t shown significant differences either way.” [citation needed]
Hunting is a behavior; now you are trying to dispute evolutionary psychology and evolutionary biology by merely discussing the presence of rods in the retina. That sort of reasoning sounds pretty shaky to me.

Also, you are making a blanket assertion that because *some* evo biologists might (you didn’t give a source) say that males are night hunters, and then disprove that assertion, therefore *all* of evolutionary biology is silly. Which is a fallacious argument, good sir.

18 Matt May 27, 2011 at 9:43 pm

Just a final note of thanks for what has been one of the best series that I have ever read on this site (or any other for that matter). Fascinating topic. Look forward to whatever you decide to pry into next to follow it. This may segue nicely into a series on logic maybe?

19 Harry May 27, 2011 at 9:49 pm

Every time I hear a politician pull out one of these logical fallacies, I wish we had a Constitutionally mandated person to follow each one of these people around to shout BULLS**T!, kill his microphone, and explain to everyone who heard the false logic why what they just heard doesn’t make sense. And team him up with a fact checker, who demands source citations on all “facts” they try to claim. Or at least that one of these people was livetweeting the presidential debates and running their commentary on the bottom of the screen on every major news network. We could use one of those guys at Town Hall meetings too.

I say ban all the misleading rhetoric and mandate only verifiable, factual, true and logical statements. Any “victory” not winnable by exclusive logos on both sides is a false victory. The best source I have found for cutting through the malarkey and getting the facts straight is NPR analyzing the issues after a debate.

That the course of nations is directed by those who most successfully abuse these fallacies is the root cause of a great many evils. Sometimes I think logos has been dying slowly ever since the Constitution was passed, and now it is old and feeble, kept alive only by a few sane people, but unable to fight off the swarm of negativity, division, and lies to gain any power.

If one side of a debate refuses to stoop to the level of the lying rabble-rouser and instead sticks to calmly refuting the other sides claims by logic, he is seen as weak, spending all his time talking about the other person, and he loses in the eyes of the public. This is why I know I could never win public office.

Also, RE: reducto ad Hitlerum (or reducto ad Nazium), there is a theory called Godwin’s Law that states that any discussion on the internet that goes on long enough will lead to someone using it, at which point the discussion is over and whoever used it has lost the argument. This doesn’t apply if it is deliberately invoked to end discussion, or if the original topic involves genocide, torture, totalitarian government, or other notable features of actual Nazis.

20 Bender May 28, 2011 at 1:16 am

Harry — then of course there is the more recent logical fallacy, Reductio ad Godwin, where someone invokes Godwin’s Law to shut down an argument when the comparison with National Socialist policies/philosophies is spot on, based on the false belief that the Nazis were so singularly evil that what they did could never, ever, ever happen again.

21 Jack May 28, 2011 at 9:40 am

Sean, in no way did I say that all of evolutionary biology is silly. You are committing your own logical fallacy there. I am merely pointing out a couple examples of the shaky, ad hoc reasoning that I’ve seen in that field.

22 lisbon May 28, 2011 at 10:27 am

Is there an illogical fallacy for one person pointing out another person’s illogical fallacy in the middle of an argument?

23 Mark Hall May 28, 2011 at 11:13 am

You know who else used reductio ad Hitlerum?


24 Tyler May 28, 2011 at 11:29 am

You guys should put all your articles on rhetorics in order on one page, so someone like me don’t have to go everywhere looking for the other articles. I don’t know know which goes after which.

25 Thomas More May 28, 2011 at 4:55 pm

Slippery Slope

Slippery slopes occur when a person asserts that a relatively small step will lead to a chain of events that result in a drastic change. Example: “If we legalize same sex marriage, what will stop us from legalizing marriage between humans and robots? Or humans and animals?”

Anyone else notice the logical fallacy used in the argument against the slippery slope argument?

26 Brett McKay May 28, 2011 at 5:48 pm

Glad you guys enjoyed the series! It was a tough one to write so it’s good to know folks found it interesting.


All the posts should be tagged now, but it will take a little while before the cache clears. I deviated somewhat from the schedule laid out in the introduction. The five canons of rhetoric were originally going to be one post, but there was so much interesting information on it that it turned into five separate posts and by the time I was done with them, I had run out of steam and the series had gone on so long I figured it was time to wrap it up.


Not much in evolutionary psychology and biology (particularly psychology) can ever be proven 100%, ad hoc reasoning or not. Maybe men have more sweat glands because they were more physical or maybe they have more sweat gland because their hairier bodies got hotter and had trouble cooling off. I don’t think we’ll ever know; there aren’t studies that can be conducted to prove either theory. But scientists can observe and use their best reasoning and then come up with possible theories. And these theories are interesting to ponder.


You’ll be able to see all the posts together by clicking the rhetoric tag at the end of the post.

27 TubbyMike May 28, 2011 at 9:18 pm

@Brett : Thank you for an amazing and interesting series. Early on in the series I posted a comment saying that I was looking forward to a post on fallacy; I wasn’t disappointed by this one. Well done sir. In this post you’ve touched on every trick in the advertisers/politicians/PR book and it’s why I’m so cynical about everything I see or hear from “official” sources. The oft overlooked point you made was for an “educated citizenry” and for me, this is the key point. Unless we strive to maintain and improve education our politicians and corporations will try to sneak any BS they can past us to maintain their positions, regardless of the veracity of their argument.

@Harry: Agree with you sir. We need an official role in politics that calls BS so that the electorate can stop and think about what they’ve just seen or heard. They can still vote for it though. That’s democracy.
Interestingly, Channel 4 News here in the UK has a factcheck service on the web-site associated with their main evening news programme. The newscaster regularly refers to it during the broadcast. I wonder how many people access it?

28 Omir May 29, 2011 at 6:30 am

My favorite example of “post hoc ergo propter hoc” is “Killing turkeys causes winter.”

29 Eudaimonia May 29, 2011 at 11:57 pm

Nice post. The difference between a formal logical fallacy and an informal “logical fallacy” cannot be understated. Formal fallacies (i.e., actual fallacies) are, with a little training, easy to spot. This is because formal reasoning is either right or wrong. However, with informal “fallacies,” there is much more gray area. For instance, there is nothing wrong with making a slippery slope argument per se, but the slippery aspect of the argument has to be demonstrated. As such, informal “fallacies” offer us guideposts and should raise tentative red flags when you notice them, but do not necessarily doom an argument like a formal fallacy would.

30 Mr Rui May 31, 2011 at 6:51 am

I’ll make up false dilemma right now, it’s my favourite

”only japanese men understand” follows ”this hobby is a pleasure” via NHK

I lack the courage to mention how he feeds his fish, in my opinion

facts: his game has mini phishing kit, with live miniature fish, set on the floor at home

I take his words and assume japanese women wouldn’t be able to do it. Since phishing wouldn’t be a problem to them, it must be something else. I thank you so much.

31 Just a thought May 31, 2011 at 12:17 pm

I get the impression that there’s some logical fuzziness (or maybe even logical fallacies!) in your article when you list the informal fallacies and then discuss the “ethical dilemma” in using informal fallacies to persuade. I think the problem is that not all of your informal fallacies are equally fallacious in the same way.

Yes, it’s true that the appeal to authority can be a fallacy: “Doctor Who is an expert in quantum physics. He says time travel is possible. Therefore, time travel must be true.” This is a logically false argument.

But the appeal to authority is not necessarily a logical fallacy: “Doctor Who is an expert in quantum physics. His study and experience in quantum physics has lead him to conclude that time travel is possible. So, perhaps time travel is possible.” This is a logically valid persuasive argument and is not a logical fallacy. There is no “ethical dilemma” about using this persuasive argument, because there is nothing logically fallacious about it.

Same with the slippery slope. Sometimes, slippery slope arguments are logically fallacious, but other times, they are logically valid if the justification for the slope is properly argued. “If we allow people to drink wine at the public park, then we won’t be able to prevent people from drinking beer also, because there is no principled justification for distinguishing between different types of alcohol.” This is a logically valid slippery slope argument — there is no “ethical dilemma” because there is no logical fallacy here.

Your same-sex marriage example is weak for that reason; as it currently reads, it is logically fallacious, but that’s just because of very poor wording. I can reword the example to make it logically valid: “If we legalize same-sex marriage, then we will be defining marriage in a way in which we won’t be able to give a principled justification for distinguishing between a person-person marriage and a person-animal marriage.” This is not logically fallacious and therefore there is no “ethical dilemma” in making this argument.

In contrast, the “strawman” fallacy is always logically fallacious. One should never argue this fallacy because it is inherently unfair or “unethical”.

In the end, your conclusion is “yes, you can use informal fallacies to persuade”. But that doesn’t make a distinction between true logical fallacies (like the strawman), and arguments that are not necessarily logical fallacies but can be logically valid (like the slippery slope and the “from authority” arguments). Good rhetoric should never use “informal fallacies” which are simply logically fallacious arguments, but it can and should use “informal fallacies” which are actually logically valid or persuasive arguments. In addition, your article also might lead some readers to incorrectly classify some logically valid or persuasive arguments as logically fallacious when they’re not.

32 JCMasterpiece (John Camiolo) May 31, 2011 at 10:09 pm

The “False Dilemma” fallacy is used all the time as a basic premise in research. Two statement are put to the test. If one is proven false, the other must be true. If one is proven true, the other must be false.

For instance: An experiment is done in which there are two study groups for an exam consisting of similar groups of people. One study group (experimental group) is given a cash incentive ahead of time to do well on a test. Meanwhile a second study group (control group) is not given a cash incentive. Both groups are given the same test and the control group has a higher average score on the test. Thus this experiment proves that cash incentives are not effective in educating students, and may in fact be a hinderance to the process.

In reality, the experimental group may have actually worked harder at preparing for the test. The experimental group may have stayed up later into the night studying and prepping. As a result they may not have gotten enough sleep and tried to compensate for that by ingesting caffeine or some other stimulant. In turn those factors caused the poor test performances. But since the research focused on the cash incentives and ignored other factors (such as a lack of understanding of healthy study skills), the experiment produced a flawed conclusion.

Unfortunately, this kind of validity issue is extremely common in the worlds of research and experimentation.

33 Nathan June 2, 2011 at 11:19 am

I didn’t see anyone mention the “No True Scotsman” fallacy. Here’s the explanation:

Two Scotsmen were sitting in the pub. One told the other of a crime that had just happened in which a little girl was killed while walking home the previous night. He also said the killer hadn’t yet been caught. The other said “The police shouldn’t be looking for a Scotsman, then. No Scotsman could’ve done this. Scotsmen are not capable of such things.”

The next night, the same two Scotsmen were in the same pub. The one who originally told the story told the other said the police had found the killer and that he was, in fact, a Scotsman. The other responded by saying “Well, no TRUE Scotsman could’ve done this.”

You see this one all the time from religious moderates. Take Muslims as an example. They say they have a religion of peace, but when one of their more radical believers does exactly the kind of thing the Koran tells them to by running a plane into a building, the moderates say that person couldn’t have been a real Muslim…or they just condone the action with silence. This is also used to justify Christian atrocities of the past.

34 Tae McNeelege June 2, 2011 at 5:28 pm

All rhetorics fail when you keep asking why. Keep asking why and the speaker will shoot himself in the foot with his own logic.

35 Jason Taylor June 2, 2011 at 10:51 pm

“You see this one all the time from religious moderates. Take Muslims as an example. They say they have a religion of peace, but when one of their more radical believers does exactly the kind of thing the Koran tells them to by running a plane into a building, the moderates say that person couldn’t have been a real Muslim…or they just condone the action with silence. This is also used to justify Christian atrocities of the past.”

That is a matter of wording. The speaker could mean by religion X,”Someone who claims membership of religion x would not do such and such”(untrue) or “someone who does such and such has indicated in so doing that his claim is theologically unsound to the point of rendering his claim meaningless.”(debateable in a given situation). Usually it is the second claim that is made, not the first.

“Scotsman” by contrast is a matter of chance, not of choice, except for immigrants. Someone could behave in any manner whatsoever and believe anything whatsoever and not cease to be a Scotsman unless his citizenship is formally revoked. Even if he is executed for murder he has not ceased to be a Scotsman-he has simply become a dead Scotsman.
Even here the speaker could simply mean, “The given person has not behaved in a manner appropriate to the ideals peculiar to Scotland.” Which begs the question of whether in fact Scotland has ideals peculiar to itself and if so what are they? But that is another story.

36 Joshua Krohse June 3, 2011 at 11:39 am

Shouldn’t this line “Therefore, no gods have beards” in the example for a negative conclusion from affirmative premises be “Therefore, no mortals have beards”?

37 Max June 8, 2011 at 5:04 am

Thanks, one of the best series on AoM, will it be in the next book or is it already included in the AoM book?

38 Justin Powers June 8, 2011 at 6:05 pm

“You know who else was a vegetarian? Hitler. Therefore, vegetarianism is bad.”

Sounds reasonable enough to me.

39 Nicholas June 13, 2011 at 12:16 am

God is Love.
Love is Blind.
Ray Charles is Blind.
Therefore, Ray Charles is God.

40 Sullivan Ballou June 14, 2011 at 6:31 pm

Many who apply the “No True Scotsman” fallacy put the subject that they are trying to trap with logic (in this case, a member of a certain religion) in a box, often by using a simple dictionary definition to define what a “true [whatever]” DOES do, and excluding everything that the basic definition leaves out for the sake of conciseness. For example, to defend Christianity from the “atrocities” of Christians in the past, one might say “No true Christian would torture and kill people,” since these actions are the most prominent atrocity by all human understanding. However, because Christians can be assumed, usually by definition, to regard what the Bible says (among this, “Love one another” and “Thou shalt not kill”), it is NOT an example of the “No True Scotsman” fallacy to state, “No true Christian” would have committed the past atrocities mentioned, simply because it CAN be logically stated that no “true Christian” would intentionally disregard what the Bible says. No one needs to rush forward to rationalize “Christian atrocities” AFTER the fact (Crusades, Inquisition, etc), because the religious laws forbidding these atrocities existed BEFORE the fact.

41 Joe June 15, 2011 at 5:04 pm

Some of my faves, though they might not all be arguments in the classical sense.

If you fought (speak out, argue) against the communists, you must be pro-nazi.

Every time they send one of those rockets into space, it rains the next day. Therefore, launching rockets causes it to rain.

I don’t even know if this qualifies as a fallacy, or just an outright falsehood, but… Head of Agency X requests a 25% increase in his budget from the legislature, legislature increases funding for Agency X by 10%, Head of Agency complains to media that his budget has been “slashed” by 15%.

I think we’re also seeing more of what could only be called something like the “double counter fallacy.” That occurs when a person whose own poor logic, sketchy data, and extreme use of fallacious argument is exposed or proven wrong, and in response proclaims himself as the victim of an ad-hominem attack.

42 Jennifer October 19, 2012 at 2:05 pm

Thank you so much for this series! I’m preparing to audition for a production of Julius Caesar, and these articles have been tremendously helpful to me in my analysis of the various speeches/monologues.

43 Hermann May 29, 2013 at 11:15 am

A true scotsman wears kilt.
I wear kilt.
Therefore I am a true scotsman!


Hermann the German

44 Sarah August 5, 2013 at 4:33 am

Foundation fallacy – A proven, B invented A, Therefore B is proven.

If the A point is proven false, than B can be said to have unstable foundation.

45 Lee October 15, 2013 at 12:23 pm

I am using this blog post as a reference to teach logical fallacies for high school seniors…

Thank you.

46 hagnat October 28, 2013 at 12:42 pm

maybe i have skipped this, but is there a logical fallacy about discrediting an argument by saying it is a logical fallacy ?

47 Jason December 22, 2013 at 3:49 pm

It depends on whether or not it is in fact a logical fallacy.

If not, then it may well be a red herring.

Leave a Comment

Previous post:

Next post:

Site Meter