Everyone longs to be in possession of reason. When we hold an idea in our inner self it is because we consider it to be the best of all possible ideas in a given matter, and we rarely give in to abandoning it without a struggle.

And that is why, when we give ourselves up to the heat of the dialectic battle, we often make use of strategies of persuasion that break with formal logic ; or we are victims of the tricks of others for taking our opinion to their terrain (and sometimes without realizing it).

This kind of juggling of logic, which very often is ignored and relegated to a second order of importance, is known as fallacies. Sometimes such fallacies are cognitive biases that harm the user more than others.

In this article we will deal specifically with the informal fallacies . Knowing them is essential to develop enough competence with our words to emerge victorious from the debates, as well as to guard against the traps that our mind (or other people) may occasionally set for us.

What are informal fallacies?

Informal fallacies are reasonings in which premises are used that could seem logical, but in reality hide an error in their own structure (the arguments are biased in their approach or allude to irrationality). Sometimes they have a sequence that is formally flawless, so that they are not easy to recognize. The aim of this text is to shed light on this issue, and to provide the necessary basis for identifying them in ourselves or in others.

Types of informal fallacies

Next we will see which are the 20 most important informal fallacies, as well as what each one of them consists of. In order to facilitate their full understanding, we will also review concrete examples.

Ad hominem

The ad hominem fallacy directly alludes to the person who pronounces an argument, but it does not at any time consider the truthfulness or logic of what he says . The aim, in such a case, is to discredit the interlocutor or to belittle his or her opinion based on “supposed” undesirable qualities, which undermine the strengths of his or her persuasive effort. For example: “if you are not a student at the university you are totally ignorant, and you have no right to give your opinion on this matter”.

2. Ad baculum

The structure of an argument is supported in an illogical way, using imposition, threat or violence in order to persuade others to carry out an action or assume an attitude as their own. The content of such messages is unfounded in any way, and often occurs in the context of a vertical or asymmetric relationship (from the one in authority to the one out of authority). For example: “this is done because I say so”.

3. Ad verecundiam

A certain thing is said to be true for the sole reason that the person saying it has a position of authority or is an expert on the matter at hand.

The prestige of the source is the only reason used to validate an argument, without considering the fact that people may make mistakes (or other types of bias) despite having deep knowledge. It is also sometimes claimed that something is true because it has been published in the media. For example, “that should be true, because it’s been said on TV”.

4. Ad populum

The general belief in the subject under discussion is used to infer from this that the position held on it is correct or true. From this fallacy it is deduced that the popular consensus undoubtedly points out what is right , so that the direction in which others think has to rise as the standard from which to orient one’s vision of things. For example: “if this song is the first one in the sales lists, it is because it must be good and worthy of being heard”.

5. Ad ignorantiam

Although the possibility of falsifying any hypothesis is a necessary characteristic for it to be considered in the field of science, this fallacy points out that the inability to prove something wrong implies that it must be true. Whoever makes use of it does not consider it important to prove the certainty of what he is affirming, but that the speaker proves its falsity . For example: “I have a lion in the garage, and if not prove that I am making it up”.

6. Ad antiquitatem

Traditions are for many people the basic guide to conduct themselves in life and in their decisions as to how to proceed in the face of daily uncertainty. Thus, what is passed on intergenerationally stands as the most basic rule, and the reason why something must be right or wrong. People who use this fallacy say that if a way of “doing things” was useful for a long time, it will continue to be useful in the present and in the future . For example: “this is because all life has been useful”.

7. Ad novitatem

This fallacy can be considered a mirror of the previous one. In this sense, will support the veracity of any argument by alluding to its novelty or to the fact that it opposes what was once considered differently. Those who use it believe that the passage of time always leads to improvement, so anything that has recently emerged will replace what has been done by tradition. For example, “technology today is so advanced that today’s films are much better than those of 20 years ago.

8. Post hoc ergo propter hoc

This fallacy is based on the misinterpretation of contiguity, in the sense that everything that happens before an event should be its reason. Although it is true that the laws of cause and effect would require the temporal (and physical) proximity of one and the other, not everything that happens in the vicinity of some event will be directly related to it . For example: “everyone shouted at the moment the teacher entered the class, so that was the reason they did it”. It has also been called a coincidental correlation.

9. Equivocal, ambiguous or antanaclastic

Polysemous words, or words with multiple meanings , are used in order to offer a reasoning whose processing transfers the subject who might receive it to very ambiguous interpretations. On some occasions it is even possible that the very connotations arising from this are so discrepant that the intention is to manipulate the listener on the basis of the ‘twisting’ of the rich semantics of a language. For example: “the end of life is only death itself” (understanding “end” as “purpose” or as “end”).

10. Straw man

This fallacy consists in taking the argument of the person with whom one interacts to its ultimate consequences, forcing him or her to assume the most extreme position possible and distancing him or her from moderation. In this way, it is possible that a considered reasoning becomes blurred and deformed, facilitating much simpler counter-arguments.

This fallacy also implies the distortion of the original purpose , until it ends up becoming a different one and difficult to defend. For example: “if you say that all people are equal, and murderers are ultimately people, then you are like all murderers”.

11. Affirmation of the consequent

Every event can be divided into its causes and consequences, in other words, into antecedents and consequences. Sometimes an event can have more than one consequence and, moreover, when the latter occurs it need not have been preceded by a single cause. This process leads to conclusions that may be true, but that do not explore all the options that could come into play . For example: “when it rains the ground gets wet. As this ground is wet, it can be said that it has rained for sure”.

12. Denial of the antecedent

This case is the opposite of the previous one. As in that case, it is necessary to break down a fact into its causes and consequences. After that, a cause would be treated as the “absolute value” for the consequence involved, omitting in this act any additional explanatory factor for the consequent one.

For example: “the person who works gets what he may want. If he does not work, then he will never achieve it” (although he could do so through other means, such as chance, or by changing his goals to others where work is not so relevant).

13. Hasty generalization

This fallacy implies that, from a series of isolated personal experiences (which are not representative of reality), the generalization of a much more complex phenomenon is carried out. It is a mechanism through which a fact that is too complicated to be fully and absolutely apprehended is cognitively simplified, and through which many times unfair stereotypes are born for those who bear them. For example: “I once went to the doctor and he missed my diagnosis, and they are all inept”.

14. Request for principle

This fallacy implies the articulation of premises that, by their very formulation, approve a series of assumptions that have not been validated in reality. And this is so because, by accepting their content, other different aspects are accepted in a secondary way that have not been able to be corroborated.

This way, if you agree with the original reasoning, you will be approving others without us really noticing . For example: “I always tell the truth” (from which it will be deduced, without solid evidence, which never lies).

15. Player’s fallacy

This fallacy distorts the real laws of probability by focusing on past events that are not really relevant . It is used, above all, in matters related to chance and has been systematically proven in those suffering from pathological gambling. It can be a problem when it encourages them to persist in the behaviour that is leading them to economic ruin. For example: “when the coin is flipped, both the heads and tails have up to a 50% chance of appearing. I’ve already done it nine times and all of them have come up heads, so it’s more likely that the next one will come up tails”.

16. Ad nauseam

The fallacy ad nauseam consists of repeating the same idea enough times to make it real for the interlocutor . It is based on the premise that “when a lie is told over and over again it ends up becoming a truth”. This is a strategy widely used in the advertising sector, from which the aim is to strengthen consumer confidence by reiterating the supposed advantages of a product or service in the different media. For example: “our toothpaste is the most effective in preventing tooth decay” (repeated on different channels and in multiple time slots).

17. Ad misericordiam

This informal fallacy resorts to pity or mercy in order to reinforce the appropriateness of what one is trying to achieve . It implies a search for reason, or interpersonal persuasion, appealing to the emotions of the interlocutor and his or her empathy with our situation. Emotions have a relevant role here, since they are manipulated by ignoring the most basic logic. For example: “Please pass the exam, you don’t know how many days I’ve spent without sleep

18. Ad consequentiam

This fallacy suggests that, if the consequences arising from a premise are negative, the premise cannot be true. Thus, the arguments are stripped of their veracity because, if they were accepted, something would be assumed with it that is not pleasant or that may even become catastrophic. As can be seen, it is very close to denial, and has a very important emotional substratum. For example: “this climate change thing is a hoax, because if it were true, in just a few centuries the planet would be in ruins”.

19. False dilemma

This fallacy aims to reduce a multiplicity of possible options to be chosen in only two, and often exclusive, alternatives. This creates an artificial dilemma in which the person is forced not only to choose from the few options offered, but also to accept without further reflection the path that others have laid out for him. For example: “either you are with me or you are against me”.

20. Ad crumenam and ad lazarum fallacy

These fallacies imply the attribution of truth to the argument by the fact that the user is rich (ad crumenam) or poor (ad lazarum). It is similar to the cognitive biases of the winner and the loser, a well-known phenomenon in Social Psychology that explains how people position themselves in favour of an individual because they are in a privileged or disadvantaged situation in a concrete competitive context (especially in politics). Thus, it focuses on resources, or the absence of them, as a criterion from which to recognize the goodness of discourses. For example: “if Bill Gates says it, it must be true”.

Bibliographic references:

  • Cummings, L. (2014). Informal Failures as Cognitive Heuristics in Public Health Reasoning. Informal Logic, 34, 1 – 37.
  • Hitchcock, D. (1989). Informal Fallacies. Teaching Philosophy, 12, 49 – 51.