An informal fallacy is a fallacy that is caused by the content and context of an argument, and not necessarily due to the form of the argument.
Scholars commonly define fallacies as deceptively bad arguments. In contemporary studies, scholars often distinguish between formal and informal fallacies (Hansen, 2020).
Formal fallacies are those arguments that have an invalid logical form. By contrast, many informal fallacies are also invalid, but their problems often arise from their content or context rather than their logical form.
The importance of understanding informal fallacies has been widely acknowledged since the 1970s (Johnson & Blair, 1993).
The discovery and definitions of core informal fallacies, as outlined in Irving Copi’s Introduction to Logic (Copi et al., 2013) as well as later introductory-level textbooks on logic, can be traced back to two primary sources:
- Aristotle’s On Sophistical Refutations (Aristotle, 1955), and
- John Locke’s An Essay Concerning Human Understanding (Locke, 1825).
What follows is a list of 18 commonly recognized informal fallacies.
The ad hominem fallacy involves attacking the arguer’s personal situation or traits.
There are three commonly recognized kinds of ad hominem:
- The abusive ad hominem
- The circumstantial ad hominem
- The tu quoque ad hominem
The first kind involves arguing that someone’s view should not be accepted because they have some negative property.
For example: “Alice’s view that breaking promises is immoral should be rejected because Alice is rude.”
The second kind involves arguing that someone’s view should not be accepted because their view is supported by self-interest.
For example: “Bob’s recommendation to exercise regularly should be rejected because Bob owns a gym.”
The tu quoque involves arguing that someone’s view should not be accepted because their actions are inconsistent with that view.
For example: “Clarence’s view that people should not drive cars should not be accepted because Clarence drives a car.”
Read More: Ad Hominem Fallacy Definition and Examples
The appeal to ignorance fallacy (Ad ignorantiam) originates in Locke’s essay (1825).
Such arguments assume that the inability of an opponent to produce a better argument is sufficient reason to think that the proponent’s argument must be accepted.
In other words, arguments that commit the appeal to ignorance fallacy assume that a proposition is true because there is no evidence against it (Krabbe, 1995).
For example: “Invisible unicorns exist because no one can prove that they don’t exist.”
The argument to moderation (argumentum ad temperantiam) is the fallacy that the truth always lies somewhere between two opposing positions.
It is also known as a false compromise, an argument from the middle ground, and the golden mean fallacy.
For example, A says that the sky is blue. B says that the sky is yellow. C incorrectly concludes that the sky must be the intermediate color green (Gardner & van Stralen, 2009).
Begging the question (petitio principii) is not a logical fallacy per se, but it is still considered an informal fallacy.
Arguments that beg the question essentially assume their conclusion.
For example: “to allow everyman an unbounded freedom of speech must always be, on the whole, advantageous to the State; for it is highly conducive to the interest of the Community, that each individual should enjoy a liberty perfectly unlimited, of expressing his sentiments” (Whately, 1836).
This argument begs the question because the premise and the conclusion are practically the same.
Arguments that commit the fallacy of equivocation exploit the ambiguity of a term or phrase that occurs at least twice in the argument.
In one instance, such a term or phrase has one meaning, while in another, it has a different meaning.
Hansen (2020) gives the following example:
- The end of life is death.
- Happiness is the end of life.
- Therefore, happiness is death.
This argument exploits the ambiguity of “the end of life.”
A false dilemma or a false dichotomy is a fallacy that unjustifiably limits the available options.
Such arguments are based on a premise that offers a false number of alternatives, one of which must be true.
For example: “Diana does not like meat; therefore, she must be vegan.” The argument erroneously assumes that Diana must be vegan because she doesn’t like meat.
False equivalence is an informal fallacy in which an equivalence between two things is drawn based on flawed, false, or incomplete reasoning.
The most familiar example of such a fallacy is expressed in the colloquial phrase “comparing apples and oranges.”
More precisely, the argument would look something like the following: “Apples and oranges are both fruits; therefore, apples are the same as oranges.”
Arguments that commit the historian’s fallacy assume that decision-makers of the past viewed events from the same perspective and had the same information as those analyzing the decision now.
The idea was first introduced by Matthew Arnold in his 1880 essay The Study of Poetry (Foundation, 2022).
For example, past military blunders seem obvious to us now, but they would not have seemed so to the people that committed them.
Such arguments involve citing an irrelevant authority as evidence of your argument.
This is a form of an appeal to authority that is made even less reliable by the fact that the authority one appeals to is irrelevant to the subject in question. For example:
- A says that eating junk food is healthy.
- A is a distinguished mathematician.
- Therefore, eating junk food is healthy.
Not only does the argument assume that what A says is true, but it also cites an authority that has no necessary competence in nutrition.
The is-ought fallacy occurs when one argues that things should be a certain way because they are that way.
For example: “We don’t currently wear face masks in hospitals so we should never start wearing them.”
The is-ought fallacy should not be confused with the is-ought problem formulated by David Hume (Hume, 1888).
Moving the goalpost involves changing the demands put on the opponent during an argument.
One may present evidence in response to a specific claim, but the opponent may dismiss it and demand greater evidence.
For example, if I feel that I’m about to lose the argument, I might try to shift the nature of the discussion to make my job easier.
The ought-is fallacy is the converse of the is-ought fallacy. It assumes that just because things should be a certain way, they are that way (Ought-Is, 2019).
This is also often called wishful thinking. For example:
- Unicorns should exist.
- Therefore, unicorns exist.
This argument assumes that an ought implies an is. But even if the premise was true, the conclusion would not logically follow.
A red herring is something that misleads or distracts from something important (Red Herring, n., 2015).
As an informal fallacy, this occurs when an argument that is not relevant to the discussion at hand.
One example of the red herring is whataboutism, where a person says “what about…” and talks about something not directly related to the debate.
Retrospective determinism is an informal fallacy that assumes that something happening is proof that it was bound to happen.
It was coined by French philosopher Henri Bergson (Anderson, 2021).
A clear example of this is a phrase like: “The Great Depression was bound to happen.” Of course, this reflects the benefits of hindsight that make up overconfident about our judgments of the causes and obviousness of past chains of events.
A slippery slope is a fallacy that occurs when one unjustifiably assumes that a small step leads to a chain of related events culminating in some significant effect.
Such arguments may sometimes be reasonable, but they are not deductively correct.
Generally, the slippery slope is an argument against change, arguing that one small change in policy or approach will set off a chain reaction that will lead to disastrous consequences.
A straw man refutes an argument that was not actually advanced by the opponent.
The arguer replaces the real position of their opponent with a weak and false version (a straw man).
This fallacy is common in political discourse, where politicians will misrepresent their opponents’ positions in order to harm them.
Read More: Straw Man Fallacy Definition and Examples
This fallacy justifies a belief or behavior by reference to the fact that others believe/do it too.
For example: “I cheated on my exam but that’s not a problem because lots of people cheat on their exams.”
We can sum this up as two wrongs don’t make a right.
Unwarranted or faulty generalization occurs when one draws a conclusion about all or many instances of a phenomenon based on one or few instances.
For example: “I have seen many white swans; therefore, all swans are white.”
The above example also happens to fit into the fallacy of division.
Informal fallacies are deceptively bad arguments that may not be formally invalid. There are many commonly recognized types of informal fallacies. Most of these can be traced back to Aristotle and John Locke. It is important to understand how informal fallacies work and what their supposed faults are.
Anderson, R. (2021, August 27). Rebel Without a Cause – Reconstructing Free Will in Bergson. Epoché Magazine. https://epochemagazine.org/43/rebel-without-a-cause-reconstructing-free-will-in-bergson/
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