What is a Fallacy?

Fallacies are mistakes of reasoning, as opposed to making mistakes that are of a factual nature. If I counted twenty people in the room when there were in fact twenty-one, then I made a factual mistake. On the other hand, if I believe that there are round squares, I am believing something that is inconsistent. This is a mistake of reasoning, and a fallacy, since I should not have believed something inconsistent if my reasoning is sound.

In some discussions, a fallacy is taken to be an undesirable kind of argument or inference. For example, a certain textbook explains "fallacy" as "an unreliable inference." In our view, this definition of fallacy is rather narrow, since we might want to count certain mistakes of reasoning as fallacious even though they are not presented as arguments. For example, making a contradictory claim seems to be a case of fallacy, but a single claim is not an argument. Similarly, putting forward a question with an inappropriate presupposition might also be regarded as a fallacy, but a question is also not an argument. In both of these situations, though, the person is making a mistake of reasoning since he is doing something that goes against one or more principles of correct reasoning. This is why we would like to define fallacies more broadly as violations of the principles of critical thinking, whether or not the mistakes take the form of an argument.

The study of fallacies is an application of the principles of critical thinking. Being familiar with typical fallacies can help us avoid them. We would also be in a position to explain other people's mistakes. There are different ways of classifying fallacies. Broadly speaking, we might divide fallacies into four kinds.

  • Fallacies of inconsistency: cases where something inconsistent or self-defeating has been proposed or accepted.
  • Fallacies of inappropriate presumption: cases where we have an assumption or a question presupposing something that is not reasonable to accept in the conversational context.
  • Fallacies of relevance: cases where irrelevant reasons are being invoked or relevant reasons are being ignored.
  • Fallacies of insufficiency: cases where the evidence supporting a conclusion is insufficient or weak. (14)

Types of Fallacies: Fallacies of Insufficiency

Limited Sampling

  • Momofuku Ando, the inventor of instant noodles, died at the age of 96. He said he ate instant noodles every day. So instant noodles cannot be bad for your health.
  • A black cat crossed my path this morning, and I got into a traffic accident this afternoon. Black cats are really unlucky.

In both cases, the observations are relevant to the conclusion, but a lot more data is needed to support the conclusion. These are hasty generalizations, based on small sample sizes of data.

Appeal to Ignorance

  • We have no evidence showing that he is innocent. So he must be guilty.

If someone is guilty, it would indeed be hard to find evidence showing that he is innocent. But perhaps there is no evidence to point definitively in either direction, so the lack of evidence is not enough to prove guilt.

Naturalistic Fallacy

  • Many children enjoy playing video games, so we should not stop them from playing.

Many naturalistic fallacies are examples of fallacy of insufficiency. Empirical facts by themselves are not sufficient for normative conclusions, even if they are relevant. (15)

Other Common Types of Fallacies

ad hominem

(Latin phrase for "against the man")

A theory is discarded not because of any evidence against it or lack of evidence for it, but because of the person who argues for it.

This often takes the form of attacks against someone’s physical appearance.


A: The Government should enact minimum-wage legislation so that workers are not exploited. B: Nonsense. You say that only because you cannot find a good job.

ad ignorantiam

(appeal to ignorance)

The truth of a claim is established only on the basis of lack of evidence against it. A simple obvious example of such fallacy is to argue that unicorns exist because there is no evidence against such a claim.

At first sight, it seems that many theories that we describe as scientific involve such a fallacy. For instance, the first law of thermodynamics holds because so far there has not been any negative instance that would serve as evidence against it. But notice, as in cases like this, there is evidence for the law in the form of many positive instances, which therefore prove its existence.

ad populum

(appeal to popularity)

The truth of a claim is established only on the basis of its popularity and familiarity. This is the fallacy committed by many commercials. Surely you have heard of commercials implying that we should buy a certain product because it has made to the top of a sales rank, or because the brand is the city's "favorite." In the middle of the twentieth century, tobacco manufacturers featured ads touting the number of doctors that approved of their cigarettes.

These appeals can be viewed as bandwagon appeals (everybody is doing it). Sometimes, they take the form of patriotic slogans ( Levi’s—America’s jeans ).

Begging the question

question (petito principii)

In arguing for a claim, the claim itself is already assumed in the premise. Example: "God exists because this is what the Bible says, and the Bible is reliable because it is the word of God."

Complex question or loaded question

A question can be fallacious if it is posed in such a way that a person, no matter what answer he or she gives, will inevitably commit him/herself to some other claim, which should not be presupposed in the context in question.

A common tactic is to ask a yes-no question that tricks people into agreeing to something they never intended to say. For example, if you are asked "Are you still as self-centered as you used to be?" then no matter whether you answer "yes" or "no," you are bound to admit that you were self-centered in the past. Of course, the same question would not count as a fallacy if the presupposition of the question is indeed accepted in the conversational context.

False dilemma

Presenting a limited set of alternatives when there are others that are worth considering in the context. Example: "Every person is either my enemy or my friend. If he/she is my enemy, I should hate him/her. If he/she is my friend, I should love him/her. So I should either love him/her or hate him/her." Obviously, the conclusion is too extreme because most people are neither your enemy nor your friend.

Non sequitur

A conclusion is drawn which does not follow from the premise. This is not a specific fallacy but a very general term for a poor or incomplete argument. Many of the examples in this section illustrate qualities of the non sequitur. The term roughly translates to "it does not follow," and it can be considered a fallacy of irrelevancy if not fully defended with reasonable premises.

Example: John has a safer car than mine because it cost $10,000 more when he bought it.

Post hoc, ergo propter hoc

(literally, "after this, therefore because of this")

Inferring that X must be the cause of Y just because X is followed by Y.

For example, having visited a graveyard, I fell ill and infer that graveyards are spooky places that cause illnesses. Of course, this inference is not warranted since this might just be a coincidence. However, a lot of superstitious beliefs commit this fallacy. Just because two events happen close to each other in terms of their timing, that doesn’t necessarily prove any causal relationship.

Red herring

Within an argument, some irrelevant issue is raised which diverts attention from the main subject. The function of the red herring is sometimes to help express a strong, biased opinion. The red herring (the irrelevant issue) serves to increase the force of the argument in a very misleading manner.

For example, in a debate as to whether God exists, someone might argue that believing in God gives peace and meaning to many people's lives. This would be an example of a red herring since whether religions can have a positive effect on people is irrelevant to the question of the existence of God. The good psychological effect of a belief is not a reason for thinking that the belief is true.

Slippery slope

Arguing that if an opponent were to accept some claim C , then he or she has to accept some other closely related claim C , which in turn commits the opponent to a still further claim C , eventually leading to the conclusion that the opponent is committed to something absurd or obviously unacceptable.

This style of argumentation constitutes a fallacy only when it is inappropriate to think if one were to accept the initial claim, one must accept all the other claims.

An example: "The government should not prohibit drugs. Otherwise the government should also ban alcohol or cigarettes. And then fatty food and junk food would have to be regulated too. The next thing you know, the government would force us to brush our teeth and do exercises every day."

Straw man

Attacking an opponent by attributing to him/her an implausible position that is easily defeated when this is not actually the opponent's position. This often takes the form of a fallacy of diversion, as a speaker might abandon his or her central argument and develop a secondary argument that might seem more "winnable." For instance, a parent might ask his or her daughter to mow the lawn. That child might argue that she has already cleaned her room, so she should be exempt from taking care of the lawn work. The point of contention in this example is the lawn work, so the other finished chores are immaterial to the argument at hand.

Example: When many people argue for more democracy in Hong Kong, a typical reply is to say that this is not warranted because it is wrong to think that democracy is the solution to all of Hong Kong's problems, or to say that one should not blindly accept democracy. But those who support democracy never suggest that democracy can solve all problems (e.g. pollution), and they might also agree that blindly accepting something is rarely correct, whether it is democracy or not. Those criticisms attack implausible "strawman" positions and do not address the real arguments for democracy. (15)


We are all guilty of brushing off these fallacies from time to time in our own discussions and arguments. In most cases, they are harmless tools in working through minor disagreements. Developing a stronger understanding of inductive and deductive reasoning and a clearer grasp of the logical fallacies will assist you in navigating these discussions and creating stronger, more convincing arguments of your own. It will also assist you with writing, researching, and developing stronger arguments in your college coursework and professional life. For more information on identifying, avoiding, and refuting flawed logic, please read the next section of this learning module, which features an essay by Dr. Rebecca Jones on various strategies for responding to arguments. (1)

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