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A beginner's guide to logic: spotting formal and informal fallacies

Updated on February 19, 2012

...or how to develop your critical thinking skills

A few weeks ago I was watching Channel 4 news here in the UK. The bulletin included an item about Barack Obama's proposed new healthcare policies. The bulletin didn't go into any detail about what these policies entailed; you just got a vague impression that he wanted to reform the American healthcare system and that there were all these right wing people trying to stop him. Then the Channel 4 newsreader went on to say something else: that there had been speculation that many people who objected to Obama's new proposals were doing so out of racism.

I was dumbstruck. Although Channel 4 had avoided making the bald statement "If you dislike Obama then you must be a racist", I'm sure that this very thought must have been implanted in a few viewers' minds.

An example of a non sequitur
An example of a non sequitur | Source

Maybe I'm a cynic, but I am sure Channel 4 did this deliberately - though what the reasons for this are, I don't know. Perhaps it was sheer journalistic sensationalism and laziness on their part - after all, it's much easier to do this kind of reporting than to actually undertake some proper research and present both sides of the argument in full.

After 44 years on this planet I can usually spot this kind of fallacious argument and can usually (though not always) analyse why it's fallacious. But there are surely many more people out there who are fine with the "spotting" stage, i.e. they know immediately when there's something wrong - but can't put a finger on why. If you come into this category, then this hub is for you.

Logical arguments

To help with the analysis, let's turn the "People who disagree with Obama must be racist" thing into a logical argument (also known as a syllogism).

(A) Everyone who disagrees with Obama is a racist.

(B) Homer Simpson disagrees with Obama

(C) Therefore Homer Simpson is a racist.

This is a piece of formal logic: statements (A) and (B) are known as premises, and statement (C) is the conclusion that you draw from the two premises.

Believe it or not, the argument above is perfectly logical, because if (A) and (B) are true, then (C) has to be true as well. (In other words, it doesn't contain any formal fallacies - more of which later.) However, formal logic alone is not enough for an argument to have real strength. I know that premise (A) above isn't true (not everyone who disagrees with Obama is in fact a racist); therefore the conclusion (C) doesn't hold water. Homer Simpson might be a racist, but we haven't proved it.

Not convinced? Try this:

(A) All cats speak Klingon

(B) Fluffy is a cat

(C) Therefore Fluffy speaks Klingon.

Again, the formal logic is perfectly valid. But if you believe that cats can speak Klingon, then I'd advise you to keep taking the tablets.

Let's go back to Barack Obama, specifically premise (A) ("Everyone who disagrees with Obama is a racist"). This faulty premise contains an informal fallacy known as "ad hominem", which is Latin for "to the man". Its basic meaning is "attack the person rather than attack the argument". You see a LOT of it on Internet forums, and you also see it in the media though it's usually less rampant and is often subtly disguised like in my Channel 4 news example.

Some more informal fallacies include:

  • Red herring - introducing a bogus argument or topic that's got nothing to do with the topic at hand
  • Bandwagon - as the name suggests, accepting something just because everybody else does
  • Appeal to authority - assuming that someone is right about a subject just because of their qualifications in that subject
  • Appeal to emotion - often used in advertising and politics, it involves generating favourable emotions in people so that they will associate a product/political party with those emotions and buy/vote accordingly. Negative emotions can also be used, to make people avoid something
  • False dilemma - presenting people with just two options/arguments, when in fact there are more than two
  • Hasty generalisation - basically, this means stereotyping (e.g. "All women are over-emotional").
  • Straw man - ignoring someone's real views and presenting a distorted version of them, and then attacking that distorted version rather than the real one.

There are loads more. Read about them here.

For most people in their everyday lives, being able to spot informal fallacies will be enough. But if you want to go a bit deeper, read on.


Formal fallacies

As hinted above, a formal fallacy is a structural flaw in a logical argument or syllogism. An example would be the following:

(A) If Fluffy is hungry, then she miaows

(B) Fluffy is miaowing

(C) Therefore she must be hungry.

This type of formal fallacy is known as affirming the consequent. Just because Fluffy is miaowing, it doesn't necessarily mean she's hungry - she could be miaowing for other reasons like having a thorn in her paw, for example.

Here is a related formal fallacy, known as denying the antecedant:

(A) If Fluffy is hungry, then she miaows

(B) Fluffy is not hungry

(C) Therefore she isn't miaowing.

Again, Fluffy could be miaowing because of that thorn in her paw, or for any number of other reasons!

As with the informal fallacies I've only scratched the surface here, but if you want to read more, this comprehensive guide will help. It's a huge subject, which I've only started to get to grips with myself (some people spend years studying this stuff). But hopefully it will help to clarify your thinking on various issues - if nothing else, you can always play a game of "spot the fallacy" next time you watch the news or participate in an Internet forum.

© Empress Felicity November 2009


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    • profile image

      sophs 8 years ago

      I love this hub! Great work, definitely bookmarking :)

    • profile image

      C.J. Wright 8 years ago

      Great HUB. Should be required reading for forum participation!LOL

    • EmpressFelicity profile image

      EmpressFelicity 8 years ago from Kent, England, UK

      LOL. I've dipped my toe in the religion forums and once or twice have been tempted to post a comment, but decided that discretion was the better part of valour... IMO it would need more than a hub about logic to persuade some people to think logically!

    • Quilligrapher profile image

      Quilligrapher 8 years ago from New York

      If only this hub could be made required reading before posting in the religion forums?


    • Niteriter profile image

      Niteriter 8 years ago from Canada

      I always enjoy structured thought; I truly enjoyed this Hub. Well done!

    • EmpressFelicity profile image

      EmpressFelicity 8 years ago from Kent, England, UK

      Hi Amanda, I'm not surprised by your saying that advertising and hypnosis/NLP are related. I wouldn't be surprised if TV scriptwriters use it as well. For some of us it's a real turn-off though. When I just *know* that someone is trying to manipulate me, it closes my mind to what they're saying so even the valid parts of their message just don't get through. Although I'm probably not as immune as I think I am LOL.

    • Amanda Severn profile image

      Amanda Severn 8 years ago from UK

      Lovely hub here, Empress. In my early working life I was involved in advertising and marketing, then, much later, I studied hypnosis and NLP. You'll probably not be too surprised to learn that advertising and hypnosis are quite closely related, and that many of the linguistic shennanigans you mention above are commonly used by those involved in both professions. It certainly makes you think!