Often times during debate the internal logic of one’s argument is constructed in a way that makes little sense. Such ways of arguing use what’s called “Logical Fallacies“: arguments that are meant to sound logical but when broken down do not actually make any sense. Thus, they are a fallacy in logic.

There are many types of logical fallacies, but there are approximately 5 that are used most often and that I see crop up all over the place. If you understand these logical fallacies, you will have a great deal of an advantage during debate. It will allow you to deconstruct other people’s arguments and will also help distinguish those that actually make logical sense from those who don’t.

Without further a-dieu,  I present to you my top five logical fallacies:

Logical Fallacy #1: The Strawman

A strawman argument is where one establishes a false belief by another and then attacks this belief, even though no one has explicitly agreed with it. It is called a strawman argument as a metaphor: the debater is arguing a false assumption that does not actually exist, rather than the real claims.

From my anecdotal experience, I would say that this argument is definitely one of the most used logical fallacies.

An example of this is when creationists say something like, “Atheists believe that everything came from nothing!” This is a strawman because no one is actually claiming that everything came from nothing. Instead, most atheists would argue that we simply don’t know where everything came from and probably will never know. Not that everything came from nothing!

Logical Fallacy #2: Argument from Authority

This one is a bit tricky. Certainly, the opinions on subjects of experts in particular fields should be valued higher than those of untrained laymen. However, experts are disproved all the time. You will find doctors who believe that vaccines cause autism, chemists who say they have developed cold fusion, and engineers who believe in perpetual motion. These are all claims that have been disproven countless times, but yet there are always cranks out there that believe in this stuff.

Worst of all, this logical fallacy is often used to boost an argument by using someone as a reference who is not even an expert in the field being discussed. For example, I recently was told that Sir Ken Robinson’s opinions on education were correct because he was knighted. Sure, that’s great, but does his status as a knight make his arguments any stronger? Not necessarily.

Another example of this is when opponents of genetically modified agriculture point to the British royal family. Prince Charles is sternly opposed to genetically modified food and his status as a prince is used to bolster his argument. How would he know whether or not genetically modified food is good or bad? Just because he is a prince does not mean he is correct about everything he says.

This logical fallacy is countered with an understanding that no ONE individual’s opinion should be used to make up your mind about anything. Rather, it is always best to look at what all the experts of a subject believe. Also, what have the studies shown? It is the complete scientific literature that should be used when constructing our beliefs.

For example, there will always be a study here or there that shows that homeopathy is useful, but 95% (at least) of the scientific literature disproves homeopathy with well-constructed studies. Therefore, it does not work.

Logical Fallacy #3: Ad hominem

Ah, yes. The ad hominem logical fallacy.This one is probably the easiest to remember (by concept, not name). An ad hominem argument is one where the debater personally attacks the other. This can be a very straight forward attack, such as, “well you are an idiot, so why should I listen to you?” Or, it can be more concealed, such as “you have no educational degree in this, therefore you’re opinion on X can’t be taken seriously.”

One of the most common ways this argument is used has been demonstrated by believers of pseudoscience when they describe someone as “close-minded” because they don’t believe what they do.

You don’t believe in the paranormal because you are a close-minded person.”

If you hear something like this, it is an ad hominem argument with no bearing on the question at hand.

Another very common ad hominem argument used by many believers of alternative medicine  is the discrediting of doctors through claims that they are in the pocket of the pharmaceutical industry, or Big Pharma as it is colloquially called. A recent example of this can be seen on the ridiculous and harmful website Age of Autism, where an ignoramus attempts to disqualify a new study linking autism to genetic defects by claiming that a scientist involved has conflicts of interest and is secretly working for a pharmaceutical company. His argument is deconstructed here on a great skeptical and medically-related blog.

Logical Fallacy #4:  Post-hoc ergo propter hoc

This is another very common logical fallacy. Its latin wording translates into, “after this, therefore because of this.”

Many people in their lives believe that something caused another thing to happen simply because it occurred before it. I have seen this from friends who have argued that the over-the-counter ‘cold-busting’ supplement Airborn helps with colds.

A cold usually goes away after a few days. When people take Vitamin C or other supplements after the onset of a cold, they get better afterwards because the cold’s natural regression had occurred. Yet countless of people believe in such ‘cures’ because they felt better not long after beginning their pseudo-medical treatments. This can lead to a vicious circle of confirmation bias, where the patient unconsciously cherry-picks positive results from his life and continues to confirm his already existing beliefs.

Another example of post-hoc ergo propter hoc reasoning is seen in those who pray for something that actually does happen. They pray that they get better, and they do. Therefore, the prayer worked. Once again, the occurrence of whatever was prayed for was only a coincidence.

Logical Fallacy #5: Ad ignorantum (The Argument from Ignorance)

This logical fallacy stems from the belief that if we don’t know something, it must have a particular reason for occurring or existing. Those who use this argument most are probably UFO-believers. The mere fact that a UFO is an Unidentified Flying Object is enough for them to believe that UFOs are alien spacecrafts.

The logic is as follows: since we don’t know what the flying object was, therefore it must have been an alien. This obviously does not hold up because it could be any number of other things that are much more likely (remember the Occam’s razor principle?). Perhaps they were military air crafts, weather balloons or a myriad of other possibilities.

Another example of this is when believers of extra-sensory perception say that humans only use 10% of their brains. Their logic is that because we don’t know what the other 90% is used for, therefore it must be, or can be, used for extra senses that are beyond our normal everyday abilities. Ignorance about something simply can not be used to make a logical argument. I should also quickly point out that we do in fact use 100% of our brains anyway.

People who argue with this fallacy don’t grasp the idea that we don’t know a lot of things and perhaps one day we will but ,even if we don’t, that does not mean that one particular unsupported answer is fact.


This is only the beginning of the many logical fallacies that exist out there. For me, too much information can be detrimental to my learning. Therefore, I thought it would be best to list only the five most common fallacies people make when constructing logical arguments. If you read this, you may begin to see these forms of logic crop up from time to time. And if you continue to make conscious notice of them, you will find, like I, that logical fallacies are inherent in our daily lives. They are everywhere.

If you are interested in reading the rest of the logical fallacies that I did not feel like mentioning, please feel free to check out Steven Novella’s summary of logical fallacies. There are many more important ones that are worth reading about and if you truly learn them, you will be equipped with a system of debating that few people have.


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