Logical Fallacies

There are a lot of logical fallacies that people commit on a fairly regular basis.  Look at any response thread on a news article or Youtube video, and you’ll see a cornucopia of faulty logic used to support claims.

As a computer programmer, I find that understanding logic is critical to my job, but I also find it useful to understand what logical fallacies are, because it’s very easy to invoke a logical fallacy to pull the wool over our eyes.  Further, it’s quite easy to commit a logical fallacy when you’re not a student of logic, and sometimes, even when you are.

I define a logical fallacy to be an argument that fails to create a sturdy bridge between claims and conclusions.  There are better definitions than that, but for my purposes, I find this definition to be abstract enough to cover most logical fallacies I see.

Deductive Errors
There are lots of ways to create a faulty bridge between claims and conclusions.  For example, consider the following:

1.  All Americans are blue
2.  John is an American
3.  Therefore, John is blue

The deductive logic in the above logical argument is perfectly good, but the bridge is still “broken” because #1 in the claim is factually inaccurate.

Inductive Errors
Here’s another logical error, this time, an inductive one:

1.  When I walked into Jack’s house, the living room had green carpet
2.  When I opened the closet, it also had green carpet
3.  Therefore, all the rooms in Jack’s house have green carpet

Where the first claim about blue Americans was a deductive error, the claim about Jack’s carpet is an inductive error.  There simply is not enough evidence to conclude that all the rooms in Jack’s house have green carpet.

Notice I’m not making any assertions about actual truth values within a claim.  It could very well be that John is blue or that all the rooms in Jack’s house have green carpet; however, support for the claims put forth in both of the above statements are not good enough to build a sturdy bridge between the evidence and the conclusion.  Therefore, the claim that John is blue, or that Jack’s house is full of green carpeted rooms is a matter of faith, not sound logic.

Multiple logical errors
Here’s another version of a common logical error:

1.  The sun rose today and yesterday
2.  Therefore, the sun will rise tomorrow

There are actually a couple errors the above claim commits.  The first is a factual error.  The sun didn’t rise.  The Earth spun in its orbital pattern, which made it appear that the sun rose.  But it didn’t rise.

The second error the argument commits is another inductive one, just like Jack’s green carpet.  There is indeed a possibility that what we call “the sun rising” will not occur tomorrow.  Again, Earth will almost certainly continue in its orbital pattern around the sun, and we will see the sun tomorrow; however, evidence of previous activity is not evidence for future activity, and this claim puts forward no compelling evidence regarding the Earth’s orbital pattern or other factors in the solar system.

Burden of Proof
Another common logic error is the matter of burden of proof, where the burden of proof is put on the wrong side of the equation, or where a claim is simply not justified with any meaningful evidence or logical consistency.  For instance, person A makes a claim that something exists without providing evidence:

John “Bigfoot is real.”
Jack: “What proof do you have?”
John: “No one has proven bigfoot is not real.  Let’s go squatching!”

In the above conversation, the burden of proof was indeed on John, mostly because he made the claim.  But consider an alternative to the above conversation:

Jack: “Bigfoot is a hoax, there’s just no way…”
John  “Bigfoot is real, dammit!”
Jack: “Show me a dead body, dwellings, DNA, and give me an evolutionary and migratory model”
John: “I don’t have to.  I saw one once on the side of the highway as I was driving.”

I think it’s reasonable to say that John still owns the burden of proof because Jack is asking for reasonable evidence that would provide compelling evidence that bigfoot exists.  In this case, Jack is helping to lead John to build a sound argument for his claim.  The only evidence John presents is his own eye-witness testimony, which Jack is right to be skeptical of – eyewitness testimony is often unreliable.

Straw Man
One of the most common logical fallacies I see is the Straw Man fallacy.  Straw man goes something like this:  Person A takes position X.  Person B says that Person A holds position Y.  Person B attacks position Y.  Here’s an example:

Jack:  “It’s not useful to make claims about bigfoot because there is no good physical evidence”
John:  “Why don’t you care about protecting animal habitats?  Bigfoots need a lot of habitat, as do other local animals, in order to survive.  Do you want all local animals to go extinct?”
Jack:  “WTF?”

John is trying to make the case that rejecting bigfoot gives one license to not care about other animals’ habitats, and that Jack’s disbelief in bigfoot is his method for disregarding overall habitat.  Of course, John’s claim is absurd.  Jack made no statement or implication about overall habitat.

Questionable Cause
Another common logical error is Questionable cause, or ignoring a common cause.  This happens when event A and event B often occur together.  The error comes when a person makes a claim that A causes B (or B causes A).  Correllation is not the same as causation.  Here’s an example:

John:  “Every time it snows, ice forms on the lake.”
Jack:  “Your point?”
John:  “Snow causes ice to form on the lake”.

Of course, we know this is an error; specifically, it ignores a common cause.  Temperature, along with dynamics in the atmosphere can effect ice formation and snow.

Begging The Question
Then there’s begging the question.  This is an error that is a result of circular reasoning, and specifically, is caused by making assumptions that are not well-supported.  For example:

John:  “I know bigfoot is real”
Jack:  “How?
John:  “The book written by Jane says so, and she’s the authority on Bigfoot”

In the above conversation, John assumes that Jane’s book is evidence enough to support the claim that bigfoot exists.  John also commits the error of appealing to authority, which is its own logical fallacy.

Appeal To Popularity
Ad Populum (Appeal to Popularity) is also very commonly committed.  In this logical error, evidence is put forward that a lot of people believe something, and therefore, that is evidence for the belief:

John:  “Do you realize how many people believe in bigfoot?  Look at all the TV shows and websites!”
Jack:  “I don’t know, John”
John:  “What is it going to take for you to believe the truth?!?”

There are a lot more logical fallacies, but these are the ones I see the most.  For more information on logical fallacies, see the below websites:



Author: Tim...Stepping Out

Tim Stepping Out

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