The Logical Fallacy & When to Reject an Argument

The word “fallacy” comes from “fallacia” in Latin which means “deceit, trick, deception.” However, the more modern definition for logical fallacies is “faulty reasoning as a result of neglecting the rules of logic” or, more succinctly, just “an error in reasoning.” These errors can be found ubiquitously throughout our society as they are often part of a bad argument. As I've bemoaned previously, bad arguments are often used by politicians, [insert product here] salesman, even your significant other to convince you that he or she needs this “much needed purchase;” they're everywhere. That said, one of the focal points of this blog will be discussing the most common fallacies that you will encounter. Last, putting together everything we've learned thus far, how do we know when an argument should be rejected?

Types of Logical Fallacies

There are two primary types of logical fallacies that you will encounter:

  • Formal Fallacies – These occur when there is a flaw in the logical structure of an argument, which renders the argument invalid for a deductive argument or weak for an inductive argument.

  • Informal Fallacies – These occur when there is a flaw with one or more of the premises of an argument, which then renders the argument unsound for a deductive argument or non-cogent (i.e., the criterion for cogency is not fulfilled) for an inductive argument.

Thus, there are two differences between formal and informal fallacies. First, formal fallacies contain a flaw in their logical structure, while informal fallacies contain a flaw in their premises. Second, formal fallacies are invalid or weak patterns of reasoning, while informal fallacies are unsound or not-cogent patterns of reasoning, but can still be either valid or strong. For a review of the various terminology that accompanies deductive and inductive argument, please reference the article “What is a Good Argument?”

An example of a formal fallacy within a deductive argument is:

  • If it's raining, then the road will be wet.

  • The road is wet.

  • Therefore, it must be raining.

Here, both of the premises are true, but there is a flaw in its logical structure, which renders the argument invalid. In particular, the premise “If it's raining, then the road will be wet” tells us that if it's raining, then the road will be wet, but that doesn't necessarily mean that if the road is wet it was absolutely due to raining. It is possible that the road has become wet for some other reason than rain, which is why we can't reach the conclusion “it must be raining” from the premises and, thus, the argument is invalid. All invalid arguments are also considered unsound.

An example of an informal fallacy within a deductive argument is:

  • I'm going food shopping at the corner grocery store and will most likely unknowingly purchase a few products that contain GMOs because they're not labelled.

  • GMOs have been shown to be unsafe to eat.

  • Ergo, I shouldn't shop at that store since I won't be able to discern between unsafe and safe food items.

The logical structure of this argument is clearly valid. That is, if both premises were true, the conclusion would also have to be true. However, as we know from science, GMOs are perfectly safe for human consumption [1]. Thus, the premise “GMOs have been shown to be unsafe to eat” is false, which renders the argument unsound.

Once more, for a deductive/inductive argument, a sound/cogent argument has a valid/strong logical structure as well as true premises. A formal fallacy indicates that the argument is invalid/weak due to a flaw in its logical structure, which also means that the argument is unsound/non-cogent as all invalid/weak arguments are also unsound/not-cogent. The informal fallacy indicates that the argument is unsound/not-cogent due to a flaw in a premise, but it still can be valid/strong.

Logical Fallacies are Not Factual Errors

It is important to note that a logical fallacy is not tantamount to a factual error. For example, suppose I recently observed a video online of how an elderly man successfully defended a store because he was carrying a side arm. From this observation, I then make the following argument:

  • Guns make everything safer.

  • Therefore, providing everyone with a gun in the home as well as a side arm will make society safer overall.

This is not a fallacious argument. However, it is factually incorrect as the premise “Guns make everything safer” is not true. While I observed just one instance of a gun saving the day, to date, a number of scientific studies have been conducted that consider a multitude of scenarios and they all point to guns making situations more dangerous for those involved [2]. On the other hand, if it were true that the evidence supported guns creating a safer environment, the conclusion would be reasonable. Overall, while the reasoning behind the genesis of the argument was fallacious, the argument itself does not contain a fallacy and is bad simply on the grounds that it contains a factual error. In other words, it has a false premise, which always makes the argument bad.

Logical Fallacies are Not Cognitive Biases

The primary difference between a logical fallacy and a cognitive bias is that a logical fallacy requires an argument, while cognitive biases are our default patterns of thinking. However, there are cases where a cognitive bias may influence an individual to commit a fallacy. For example, let's reuse the scenario from above but tweak the argument slightly. The new argument is:

  • Any new gun laws will prevent responsible gun owners from owning guns and possibly saving peoples' lives.

  • Therefore, we shouldn't allow any new gun laws to be passed.

Here, I am falling subject to what's known as the availability heuristic, which is a mental shortcut (i.e., a cognitive bias) that relies on immediate examples that come to a person's mind when evaluating a specific concept, topic, method or decision. I have just observed a man using a concealed gun to possibly save the lives of innocent civilians. Thus, primed with this observation, which causes me to fall victim to the availability heuristic, I then immediately craft an argument about guns, which contains a logical fallacy. Specifically, I have committed the slippery slope fallacy, which is that I have wrongly assumed that passing a single gun law will automatically lead to the restriction of responsible gun owners from owning any guns and possibly saving lives. The logical form of this fallacy is:

  • If A is allowed to happen, it will automatically lead to Z (an extreme hypothetical).

  • Therefore, we cannot allow A to happen.

This fallacy is a form of appealing to emotion, where the emotion in this case is fear. The cognitive bias clearly contributed to me committing a logical fallacy in my argument. However, if I just proceeded to walk around with a more pro-gun world-view immediately after viewing the video, I wouldn't be committing a fallacy, but I would still be in the grips of the availability heuristic. Once more, a logical fallacy can only be committed in an argument.

When to Reject an Argument

You are now familiar with the various types of arguments (i.e., inductive versus deductive), what makes a good argument (i.e., either a sound deductive or a cogent inductive argument), and the concept of a logical fallacy; therefore, we can move on to discuss when an argument should be rejected. Clearly, we shouldn't reject arguments that can be classified as good, but when exactly should we reject an argument? For this, there are two primary conditions:

  1. The argument fails to meet all the criterion to be classified as good. In other words, the argument is bad.

  2. The argument contains a logical fallacy.

Note, any requirement which fails to meet the criterion for a good argument is classified as a bad argument. While we have already discussed the details surrounding the first point, the second point requires some more explaining. The logical fallacy – there are many and we will discuss the more common types in the future – is of the formal or informal variety and either will render the argument unsound or non-cogent. This means that the argument is bad and, subsequently should be rejected.

If we revisit the argument used previously:

  • Any new gun laws will prevent responsible gun owners from owning guns and possibly saving peoples' lives.

  • Therefore, we shouldn't allow any new gun laws to be passed.

Again, we acknowledge the premise contains the slippery slope fallacy, which renders the argument bad and must be rejected. Let's revisit another argument from a previous post:

  • All humans are dogs

  • No dogs are animals with scales.

  • Therefore, no humans are animals with scales.

This argument is deductive in form, but the first premise “All humans are dogs,” is patently false. Thus, this false premise renders the argument bad and should be rejected. Let's try one more:

  • Most people that I know by the name of John are mean.

  • Your friend's name is John.

  • So, he's probably mean.

This is an example of an inductive argument where the premise “Most people that I know by the name of John are mean” is taken to be true. However, this isn't an adequate explanation for why all the John's have a higher probability of being mean. You are only exposed to a very tiny fraction of people by the name of John who live throughout the world. I would classify this argument as weak since, in my opinion, there is a less than 50% chance that the conclusion follows from this premise. That said, since the argument is weak, it cannot be cogent and is thus classified as bad; subsequently, the argument must be rejected.

Note, by “reject,” I mean that you inform your opposing interlocutor that he/she has failed to form a good argument for [insert legitimate reason here] and that they must either reformulate a new argument or capitulate. It is imperative that the reason for rejection be made lucid for all parties involved and discussed to make sure that everyone is in agreement. Hopefully, this doesn't further snowball into arguments within arguments; it is important to remain rational here.

It is also important to keep in mind that when responding to fallacious reasoning, dismantling the logic behind your opponent's reasoning and pointing out the flaw(s) may not be effective all the time. In practice, human interactions are highly complex within debates and go beyond the exchange of logically sound/cogent arguments with one another. When this does happen, don't just walk away because your opponent is being stubborn; you must change your approach in order to get your point across. This new strategy can be anything from avoiding logical jargon altogether and explaining the flaw in the simplest of terms to modifying your own argument to address the fallacy without explicitly stating that a fallacy has been committed.

Last, under no circumstances is the rejection of the argument to be used in its own argument against the conclusion. If this were to happen, you would in fact be committing a logical fallacy yourself. For example, let's once more revisit the gun argument from before. Let's suppose the argument is made by me and you notice that I have committed the slippery slope fallacy within my argument. You then rebut with:

  • You have committed the slippery slope fallacy here.

  • Therefore, you're wrong.

This is actually a fallacious argument on your part as you cannot use the instance of a logical fallacy within my argument as grounds for arguing against my conclusion that “we shouldn't allow any new gun laws to be passed.” A more appropriate discourse would be to inform me that I had indeed committed a logical fallacy and that I should draft a new argument as this one is bad and should be rejected. Again, rejecting an argument is not tantamount to saying that the conclusion is wrong.

Bad Argument.png


Understanding what a logical fallacy is and how it differs from factual error as well as cognitive biases is an essential step towards critical thinking. Unfortunately, they are used by many throughout our society in an attempt to persuade their audience, which is why it's important that you familiarize yourself with them. As you are now aware, a fallacy within an argument renders it bad and subsequently must be rejected. Moreover, whether it is a fallacy or a bad argument in general, the argument must be rejected in favor of a good argument. It is up to you to not only reject the bad arguments of others and ensure a good argument be presented, but it is paramount that you hold yourself to the same standards.


[1] The American Association for the Advancement of Science. Statements by the AAAS Board of Directors on Labeling of Genetically Modified Foods. 20 October 2012.

[2] Moyer, Melinda Wenner. More Guns do Not Stop More Crimes, Evidence Shows. Scientific American; October 1, 2017.