About the Author

Chris Shiflett

Chris Shiflett is an author and speaker who leads the web application security practice at OmniTI.


Logic

I often get distracted when following discussions online due to the abundance of flawed logic. It's distracting enough that I sometimes find myself tending to disagree with someone whose argument is illogical, even if I agree with the conclusion. (I can usually overlook poor grammar, as long as it's not too bad; we all make mistakes.) The following logical fallacies are the ones that I notice most often:

In case you're a pedant, note that logical fallacy is "often used more generally in informal discourse to mean an argument which is problematic for any reason, and thus encompasses informal fallacies as well as formal fallacies." Hopefully you can tolerate such informal discourse. :-)

Affirming the Consequent is something I first remember hearing about in high school, when a math class took a brief foray into logic. I can still remember the example, because it made the fallacy quite obvious:

  1. If A is a dog, then A is an animal.

  2. A is an animal.

  3. Therefore, A is a dog.

This was contrasted with a logical conclusion:

  1. If A is a dog, then A is an animal.

  2. A is not an animal.

  3. Therefore, A is not a dog.

A good example can be found in an old post of mine, Top X List of Mac OS X Annoyances. Although I don't mention Windows and only focus on my experience with Mac OS X and Linux, many people commented under the assumption that I use Windows. Now, I can't be absolutely sure how people came to this false conclusion, but I can speculate that their logic went something like this:

  1. Those who use Windows find some characteristics about Mac OS X annoying, but only because those characteristics are different.

  2. Chris finds some characteristics about Mac OS X annoying.

  3. Therefore, Chris uses Windows. (A related conclusion is that my opinion isn't valid, because I use Windows.)

Next on the list is the Ad Hominem Argument, likely the most popular online fallacy. If you've ever witnessed someone resorting to personal attacks, you know what I'm talking about. I always attribute personal attacks to the lack of a valid, logical argument. (I'm almost always right about this, too.) The false premise upon which this fallacy is based is that by attacking someone who holds a certain position, you attack the position itself.

Last on the list is my personal pet peeve, the Straw Man Argument. This is often described as putting words in someone's mouth, but more specifically, it's when you misrepresent someone else's position in order to make it seem as if your position is superior.

What bothers me about the Straw Man Argument is that it is both very dishonest and very effective. This is a rampant problem in politics, at least here in the US. We often hear:

My opponent would have you believe...

In most cases, whatever follows is not at all what the opponent wants you to believe. Lies are nothing new in politics, but this particular variety is extremely effective. (The Straw Man Argument is very popular on FOX News.)

Got any more to add to the list?

If you want to learn more, logicalfallacies.info looks like a good resource.

About This Post

Logic was posted on Tue, 18 Sep 2007 at 00:13:31 GMT.

7 Comments

1. Ben Sytko's GravatarBen Sytko said:

I agree too that a lot of people online use flawed premises and weak supporting statements to build an argument on. Even if you know a little bit of logic, you can quickly turn any argument to your favor. Great post!

Tue, 18 Sep 2007 at 02:05:03 GMT Link


2. Chris Shiflett's GravatarChris Shiflett said:

Thanks very much, Ben. I'm glad you liked it.

Tue, 18 Sep 2007 at 02:13:20 GMT Link


3. jay rex's Gravatarjay rex said:

i find - mostly in retrospect - that i sometimes sin as well (, but only if i really believe i am right, hehe).

i find it hard to keep my 'logical radar' up all the time, especially in hot discussions.

any tips on preventing yourself from resorting to these "(sl)easy" practices?

is it mostly about keeping your cool?

Tue, 18 Sep 2007 at 08:45:17 GMT Link


4. Rob Young's GravatarRob Young said:

Syllogism is a word, rubbish is a word therefore syllogisms are rubbish

Tue, 18 Sep 2007 at 08:51:38 GMT Link


5. John J's GravatarJohn J said:

This site is largely focused on the use of logical fallacies in science, but they apply anywhere. The Skeptic's Guide to the Universe has a good list of 20 common fallacies at http://theskepticsguide.org/logicalfallacies.asp My "favorite" is the post-hoc ergo propter hoc. You see it a lot and its really hard to argue against.

Tue, 18 Sep 2007 at 14:56:16 GMT Link


6. Sean Coates's GravatarSean Coates said:

Another type of logical fallacy that is rampant in online discussion is ignorance of causality.

Take the following statements:

- We have flowers in our garden. In the summer time the flowers bloom. In the winter, they don't. Therefore, blooming flowers make the sun shine and the temperature rise. It only makes sense.

Obviously, we know this to be false, but without an understanding of the cause/effect, in this scenario (ie, if you didn't know that the sun causes the flowers to bloom, and not that blooming flowers cause the sun to shine), the above is plausible.

S

Wed, 26 Sep 2007 at 01:47:04 GMT Link


7. Chris Shiflett's GravatarChris Shiflett said:

Yeah, that's a good example, Sean. I've witnessed people trying to argue that a correlation between two distinct sets of data necessarily implies that one causes the other.

More information about this specific case is available on Wikipedia:

http://en.wikipedia.org/wiki/Correl...imply_causation

Thu, 27 Sep 2007 at 02:51:28 GMT Link


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