4 Informal Fallacies

Cassiano Terra Rodrigues

As we have seen in previous chapters, one important feature of an argument is whether it is valid or not (in the case of deductive arguments), or if it’s strong or weak (in the case of inductive and abductive arguments). This chapter outlines some of the important mistakes that can be made within arguments, ensuring they are either invalid, unsound, or weak within a determined context. Within philosophy, such mistakes are called . Particular focus here will be concentrated upon informal fallacies; that is, mistakes not exclusively related to the logical form of the argument, but including also its content. This means even deductively valid arguments can still be interpreted as fallacious if their premises are deemed unjustified for whatever reasons, including rhetorical reasons (Walton 1995).

Committing flaws in reasoning is in fact very common. Sometimes fallacies just pass unnoticed. But sometimes they are intended, whether because the arguer is uninterested in being reasonable or wishes to induce someone else to make a rational error. The importance of studying fallacies then appears: without being able to identify flaws in reasonings, we would accept—or refuse to accept—any conclusions without good reasons to do so, and would have to base our beliefs purely on the trust of others. A common practice of course, but is it reliable?

More than just identifying flaws, the primary purpose of studying fallacies is to avoid falling foul of them. By showing why and when a certain way of reasoning does not support the truth of the conclusion, that is, does not offer enough convincing evidence for it, the study of fallacies becomes inescapable. Further, identifying these fallacies requires more than relying upon formal logic, it also involves a good deal of discourse analysis. That is, we are required to ask key questions related to the content of the relevant arguments: Who speaks? To whom? From which perspective? With what purpose? For this reason, the study of fallacies must take into account not only failures in logic, but misuses of argumentative techniques. What is argumentatively appropriate in one context may not be in another. The appropriateness will depend on, among other things, the purpose of the argument and the intended audience.

None of this means, however, that we cannot develop general standards for when we ought to recognise good reasoning and bad reasoning. Indeed, as has been noted in previous chapters, it’s of paramount importance that we can provide understandable and publicly accessible standards for evaluating  all manner of arguments and reasoning. Let us pay attention to three basic characteristics of good reasoning:

  1. A good argument is logically well-framed. This is the minimum requirement: the premises of a good argument offer reasons for the conclusion. However, different individuals can have different ideas about what counts as a good reason or not—good reasons for one person can be inadequate for another. So, while necessary, this requirement isn’t sufficient.
  2. As there may be disagreement about the premises, a good argument starts from acceptable premises, or premises that are warranted, and not only for the reasoner, but mainly for the audience. Of course, even though not true or plausible at all, certain premises may be acceptable, depending on the audience or even on the function of the argument in a given context. Considerations of form and content necessarily have to be taken together then.
  3. The premises must contain relevant information for the conclusion—if not all that is relevant, at least enough to make the conclusion acceptable. Concealing relevant information is a well-known form of deceiving people, just as taking certain information for granted when it has been widely contested is a mistake.

Fallacies contain errors in one or more of the senses given above. Of course, there are uncountable reasons for accepting a conclusion, such as social, cultural, and psychological reasons. However, the criteria for identifying good arguments are nevertheless logical criteria—that is, they are rational criteria, publicly open to evaluation. So, anyone could identify fallacies by paying attention to the following:

  1. Do the premises support the conclusion, or only offer very weak support for the conclusion?
  2. Are the premises well-supported?
  3. Do the argument’s premises include all the important relevant information?

To avoid being fallacious, an argument must be able to answer all of these questions in the positive. Bearing this in mind, we do not need to attempt to provide an exhaustive list of each and every possible fallacy. All we must do is learn how to identify when and how those criteria are not met, so we can understand when and how arguments fail to be good. So, let us examine a taxonomy of fallacies, that is, how they are classified, and then a list of some common fallacies.

Taxonomy of Fallacies

Our taxonomy of fallacies aims to categorise fallacies into distinct groups, highlighting the distinctive problems that members of each group possess. Our most general division is the above mentioned distinction between formal and informal fallacies. As mistakes in the form of deductive arguments have already been covered in Chapter 3, in this chapter we focus on mistakes of the second kind: informal fallacies.

Informal fallacies are so called because their errors lie not in their logical form. Instead, to appreciate what is wrong with them, we must look at the argument’s content, and thus we must examine if the reasoning within the argument meets our other criteria presented above—relevant information and acceptable premises. Such informal fallacies are normally divided into the following three general categories (Kahane and Tidman 2002, 349):

  1. Relevance fallacies: Fallacies of this kind do not present relevant information, or present irrelevant information for the conclusion.
  2. Ambiguity fallacies: Such fallacies employ unclear or equivocal terms or propositions, so that it becomes impossible to grasp a precise sense of what is being argued for. One may be led to think there may even be no sense at all, due to the indeterminacy of meaning.
  3. Fallacies of presumption: In such flawed reasoning, the conclusion rests upon certain assumptions not explicitly stated in the premises. Such assumptions are false, or at least uncertain, implausible or unjustified, so that the premises do not strictly support the conclusion. Explicating the lurking assumption usually suffices to demonstrate the argument’s insufficiency, either due to a lack of relevant information or unacceptable premises.

Common Informal Fallacies

The following list is not exhaustive and presents only some of the more common fallacies, for the sake of illustration. They are intentionally not classified according to the classification above—this is a task for you to accomplish after reading this chapter, as an exercise (there is another one at the end of the chapter, and few questions you should answer here and there). Tradition dictates the names are presented in Latin, some of which are more famous than the vernacular.

Argument directed to the person (Argumentum ad hominem)

This fallacy consists in attacking the person instead of treating the argument that the person is proposing. Consequently, the character or the personal circumstances of the speaker is raised to invalidate his or her arguments, rather than any fault identified with the argument itself. This is a very common fallacy, of which there are various forms. It will be useful to highlight two of them:

  • Offensive ad hominem. This form of ad hominem consists in calling into question the moral character of the speaker, thus attempting to dismiss the trustworthiness of the person rather than showing the actual mistakes in their arguments. The offensive ad hominem dismisses a certain opinion on the grounds that those who sustain it are to be dismissed, whatever the independent qualities of the opinion.
  • Circumstantial ad hominem. The personal circumstances of one who makes or rejects a claim are irrelevant to the truth of what is claimed. This fallacy ignores this important fact by attempting to undermine someone’s argument on the basis of their background, or current circumstances. For example, one might try to argue that we ought not listen to another’s argument as they will benefit from the conclusion’s truth. Such an appeal would obviously be unjustified.

A Question for You!

Can you think of a situation in which it would be acceptable to disregard someone’s evidence due to their personal circumstances? (Clue: think of courts of law)

The Straw Man fallacy

This is a very common fallacy. According to the principle of charity in argumentation analysis, the strongest interpretation of an argument should always be preferred. The straw man fallacy is the direct refusal to adhere to this principle, and consists in reducing an argument to some weaker version of it simply in order to strike it down. The original strength of the argument is thereby missed and, reduced to a caricature, can be easily refuted. The fallacy’s name comes from the fact that a straw man is easier to beat down than a real man. Some vegan activists claim their opponents often commit this fallacy by stating that if vegans have so much respect for animal life, they should accord the same respect to plant life as well. Vegans may justifiably claim this as a misrepresentation of their own position, and thus does not diminish its legitimacy. The straw man fallacy differs from the ad hominem fallacy in that it does not attempt to undermine the argument by directly attacking the person.

Appeal to power or threat of force (Argumentum ad baculum)

In Latin, “baculum” means a cudgel, bat or stick for hitting. An argument with a cudgel is then an appeal to brute force, or a threat of using force instead of reasoning in order to ensure one’s conclusion is accepted. The ad baculum is a sort of intimidation, either literally by physical power or any other kind of threat, so someone feels constrained to accept the conclusion independently of its truth. When someone threatens to use force or power, or any other kind of intimidation instead of reasoning and arguing, one indeed abandons logic. This can then be taken as the utmost fallacy, the most radical way of trying to impose a conclusion without reasoning in favor of it.

Think, for instance, of when someone raises their voice as a form of intimidation to force the acceptance of a conclusion, without giving reasons. A historical example of this fallacy comes from the El Salvador guerrillas’ use of a slogan in the 1980s, in order to prevent people from voting: “vote in the morning; die in the afternoon” (Manwarring and Prisk 1988, 186). The threat, of course, need not be overtly stated. In cinema, one of the most famous lines of Don Corleone, the Mafia character played by Marlon Brando in Francis F. Coppola’s The Godfather (1972), is: “I’m gonna make him an offer he cannot refuse.” One has to watch the movie to see why this is an ad baculum.

Begging the question (Petitio principii)

This fallacy arises when the argument’s premises assume the truth of the very conclusion they are supposed to be providing evidence for, so that in order to accept the premises one has first to accept the conclusion. As in such cases the conclusion acts as a support for itself, the Latin name “petition of the principles” is thereby explained. Such arguments are fallacious because they are useless in establishing the truth of the conclusion, even if ultimately the argument’s premises are true and the argument is definitely valid. Why then is this type of argument fallacious? Well, we desire independent evidence for our conclusions. After all, if we already knew the conclusion was true, we wouldn’t require an argument to prove it. Arguments that beg the question, however, provide no such independent evidence. Would you justify your statements just by rephrasing them?

Arguments that beg the question, then, are troublesome because they pretend to be providing independent evidence for the conclusion when in reality they are simply restating the conclusion, or assuming its truth, within the premises. For instance, when someone argues men are better than women in logical reasoning because men are more rational than women, this is to beg the question. Now, if being logical just means being rational, then what has been said is just that men are more logical because they are more logical. Thus the argument simply assumes the very point it is attempting to demonstrate.

A Question for You!

Can you spot some examples of this fallacy? And can you tell when a circularity in reasoning is not a fallacy? Explain.

Appeal to popular opinion (Argumentum ad populum)

The Latin means more precisely “appeal to the populace.” This fallacy consists in the mistake of assuming an idea is true just because it’s popular. Such arguments are fallacious because collective enthusiasm or popular sentiment are not good reasons to support a conclusion. This is a very common fallacy in demagogic discourses, propaganda, movies, and TV shows. Think, for instance, of marketing campaigns that say “products of brand x are better because they are good sellers.” Or when someone says: “everyone agrees with this, why don’t you?” But the “this” can be false even if everyone thinks it is true. The image below illustrates nicely this fallacy:

Historical photo of a group of men performing the Nazi salute in Nazi Germany. One person is refusing to do so and standing with his arms crossed.

Relying solely on the popularity of a person, movement or idea can have significant repercussions for society, as this photo taken in Hamburg (Germany) in 1936 during Nazi rule demonstrates. One person in this photograph, unlike the others, is refusing to perform the Nazi salute. Can you spot them? To find out about the history of this photo and its significance, see the Wikipedia page on August Landmesser.

August Landmesser Almanya 1936, via Wikimedia Commons. This work is in the public domain.

Appeal to pity (Argumentum ad misericordiam)

This happens when someone appeals to the audience’s sentiments to compel support for a conclusion without giving reasons for its truth. A clear example of this fallacy is provided by Patricia Velasco: “[I]t is not uncommon to find students who appeal to the teacher’s sentiments in order to obtain, for instance, a grade review, by reciting an unending roll of personal problems: dogs are sacrificed, marital engagements are broken, grandmothers are hospitalized” (Velasco 2010, 123).

In courts, this kind of fallacy is common, as when the humanitarian sentiments of the jury are appealed to without discussing the facts of the case. There is a very famous and peculiar case of a youth who murdered his mother and father, and then had his attorney plead for a lighter penalty claiming the youth had become an orphan (Copi, Cohen & McMahon 2014, 115).

Sometimes the evocation of sentiments is not fallacious. It can be perfectly reasonable, for example, to combine reasons for a conclusion with an appeal to outrage or anger towards a certain action. This fallacy occurs when appealing to emotions absolutely replaces giving reasons—aiming at persuasion through eliciting emotions solely, without attempting to rationally support the conclusion—so that sentimentalism is used to produce the acceptance of the conclusion, no matter what is true.

Appeal to ignorance (Argumentum ad ignorantiam)

This fallacy consists in assuming that the lack of evidence for a position is enough to demonstrate its falsity and, inversely, the lack of evidence for its falsity is enough to entail its truth. This is a very simple fallacy, for we cannot assert the truth of a proposition based on the lack of proof of its falsity, and vice versa. Lack of evidence is a flaw in our knowledge, and not a property of the claim itself. For instance, to say extraterrestrials exist because there is no proof of their non-existence would be to neglect the fact there may be no independent positive evidence for their existence either. The rational attitude to have when we have no evidence for either position is to suspend judgement on the matter.

A Question for You!

Can you imagine contexts in which ad ignorantiam is not a fallacy? Can you explain from your examples why it is not a fallacy?

Appeal to authority (Argumentum ad verecundiam)

These are arguments based upon the appeal to some authority, rather than independent reasons. We identify it when the speaker starts to cite famous “authorities,” dropping names instead of giving his or her own reasons, thus recognizing his or her own incapacity to establish the conclusion of the matter at hand, as if saying: “I acknowledge my ignorance, there are others who know better than me on this subject.” This explains its Latin name: “argumentum ad verecundiam,” which is more properly translated as argument based on modesty, or coyness, referring to the speaker, who invokes an authority to support their case.

Notice that an appeal to authority can be legitimate if the authority invoked really is an authority on the subject. If you think of citing Hegel in discussing matters of philosophy, or Marie Curie in chemistry or physics, then the appeal could be reasonable. But invoking Marie Curie’s ideas when talking about football, for instance, would in all likelihood be irrelevant. In other words, an appeal to authority becomes illegitimate when instead of giving reasons and constructing an independent inference for the conclusion, someone seeks to base a conclusion on the say-so of a putative authority, even though this someone is not a competent authority on the subject under discussion. The appeal then is fallacious. But even the highest authority’s opinion on some subject is not enough by itself to establish a conclusion. No conclusion is true or false just because some specialist has said so. Rather, one’s appeal to the word of the authority is merely a shorthand for, “they will be able to provide you with independent support for my conclusion.” If they cannot, then the conclusion is not supported by your appeal to their authority, whatever you say.

This fallacy may seem awkward, but it is in fact very common. For instance, the ideas of Charles Darwin—a renowned biologist—are not rarely invoked in discussions about matters of morals, politics or religion, without biology being really relevant to the case.

A Question for You!

Can you find other examples of this fallacy? What warrants legitimacy to an authority—community consensus? Expertise? A combination of both? What else?

Advertisement for Camel cigarettes that says more doctors smoke camels than any other cigarette.

This advert for Camel cigarettes from the back cover of Life magazine (11th Nov., 1946) relies upon the health expertise of doctors to extol the virtues of a particular brand of cigarettes. The intended effect on the audience is to make them believe that, as knowledgeable advocates of good health, doctors would not implicitly recommend a cigarette that was bad for you. The appeal to a doctor’s own actions, nonetheless, is unjustified in this case. Why? Firstly, simply because an individual does something (such as smoke a cigarette brand) does not mean they recommend it for your health, even if they themselves are knowledgeable about its effect. People engage in many unhealthy and irrational activities in their private lives. Further, the advert relies on the presumption that the doctors themselves were informed on the health impacts of cigarettes. Remember, an appeal to authority figures is only justified if those authorities actually are much more informed on the relevant matter. For the history behind this, and similar adverts, see the “More Doctors Smoke Camels” advertising campaign information from the University of Alabama.

Camel Advertisement by R.J. Reynolds Tobacco Company. Published in Life Magazine, November 11, 1946. Via the University of Alabama. Used under fair use.

Hasty generalization

This fallacy is committed whenever one holds a conclusion without sufficient data to support it. In other words, the information used as a basis for the conclusion may well be true, but nonetheless unrepresentative of the majority. Some widely known generalizations are unjustified for just this reason, such as “all Brazilians are football lovers,” “atheists are immoral people,” and “the ends justify the means.” Such generalizations are based on an insufficient set of cases, and cannot be justified with only a few confirming instances.

Our beliefs about the world are commonly based on such generalizations. In fact, it is a hard task not to do so! But that does not mean we should accept such generalizations without examination, and before seeking enough evidence to support them.


This is one of the most common fallacies. Whenever a term or expression appears with different meanings in the premises and in the conclusion, the fallacy of equivocation occurs. In these cases, the speaker relies upon the ambiguity of elements of language and shifts their meaning throughout the argument, forcing the audience to accept more than is entailed by the argument when any one fixed meaning is given to the relevant terms. A classical example is:

  1. The end of a thing is its perfection.
  2. Death is the end of life.
  3. [latex]/ \therefore[/latex] Death is the perfection of life.

Here, “end” can mean “goal” or “termination,” so the conclusion could be that the goal of life is perfection, or that life is perfected only when it is terminated. Apart from metaphysical considerations, the argument is only apparently valid, since the change in meaning and context make at least one of the premises or conclusion false (or, implausible).

A Question for You!

Can you rephrase the argument to make the fallacy clear?


Exercise One

For each statement identify the informal fallacy.


Incest must be immoral, because people all over the world for many centuries have seen it as immoral.

Answer: This is an appeal to popular opinion (and, in particular, tradition) to suggest that a particular act is immoral when, unless one makes the additional argument that morality is nothing more than the accepted norms within a society, popular opinion is no evidence at all for the claim that an act is moral or immoral.

  1. It’s not wrong for newspapers to pass on rumours about sex scandals. Newspapers have a duty to print stories that are in the public interest, and the public clearly have a great interest in rumours about sex scandals since when newspapers print such stories, their circulation increases.
  2. Free trade will be good for this country. The reason is patently clear. Isn’t it obvious that unrestricted commercial relations will bestow on all sections of this nation the benefits which result when there is an unimpeded flow of goods between countries?
  3. Of course the party in power is opposed to shorter terms, that’s just because they want to stay in power longer.
  4. A student of mine told me that I am her favorite professor, and I know that she’s telling the truth, because no student would lie to her favorite professor.
  5. Anyone who tries to violate a law, even if the attempt fails, should be punished. People who try to fly are trying to violate the law of gravity, so they should be punished.
  6. There are more Buddhists than followers of any other religion, so there must be some truth to Buddhism.

Exercise Two

Now try to find your own fallacies, both those types discussed and new ones. Here are some other types of fallacies to get you started. First, ascertain the fallacy, and then identify cases of it:

  • False cause (two kinds: non causa pro causa and post hoc ergo propter hoc)
  • Converse accident
  • The player fallacy
  • Loaded question
  • Irrelevant conclusion (ignoratio elenchi)
  • False analogy
  • Poisoning the well
  • Complex question (two kinds: composition and division)
  • Slippery slope


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Informal Fallacies by Cassiano Terra Rodrigues is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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