An informal fallacy is a type of fallacy
that is not an error in the form (or structure) of the argument.
Fallacies that involve errors in an argument's form are called
Aristotle was the first to identify fallacies.
In On Sophistical Refutations he identified thirteen,
divided into two groups: Linguistic fallacies and non-linguistic fallacies.
Since then, over 100 fallacies have been identified. The study of informal
fallacies is important to the field of informal logic.
Informal fallacies may be classified in various ways. One classification scheme involves three general categories:
- Material fallacies, which relate to the matter of the argument. These include arguments whose premises do not provide the required evidence, or which are irrelevant to the conclusion asserted.
- Linguistic fallacies, which relate to language-related defects in arguments. These fallacies may involve the use of words or sentences with vague, unclear, or multiple meanings or other inconsistencies.
- Relevance fallacies. Arguments that commit one of these fallacies play to our emotions (fear, guilt, pity, loyalty, bias, etc.) instead of drawing conclusions rationally.
Here is a list of some common informal fallacies:
- Appeals to Authority, Popularity, Force, and Pity
- The Appeal to Authority uses the opinion of an
authority or famous person to support an assertion. This line of
argument is not always incorrect; for most things in our lives, we
have to rely on the word of an authority. The fallacy lies in using
an authority in place of arguing based on the subject matter, or
taking as authoritative a statement that might not be authoritative.
- Joe said that 2 + 2 = 4, and he has an
advanced degree in mathematics. Therefore, 2 + 2 = 4.
- The Earth is flat, since many philosophers in the Middle Ages
believed that it was flat.
- Mr. X said that September has 31 days [when he was having a bad day, perhaps].
Therefore September has 31 days.
The Appeal to Popularity is a fallacy that asserts
that an assertion is correct based on its support by a large group of
people. For example: "The movie Harry Potter And The Philosopher's
Stone grossed a record amount of money in its first week.
Therefore, it is a very high-quality movie".
The Appeal to Force is committed when the arguer
resorts to force the acceptance of a conclusion by threating someone
with harm, either directly or indirectly. For example:
- If you don't believe that Margaret Thatcher was the greatest
British Prime Minister of the 20th century, I'll give
you a failing grade in the course.
- You should stop eating hamburgers, or else you'll get Mad Cow Disease.
The Appeal to Pity is in a way the opposite of
the Appeal to Force. It suggests that harm will come to the arguer
unless a certain conclusion is accepted.
- Fallacy of Collective Inference
- This fallacy is related to appeal to authority. It occurs when several different authorities are used to support premises in an argument, but where no one authority would agree with all of the premises or the conclusion.
- Argument from Ignorance
- This fallacy occurs whenever someone argues that something must
be true because it has not been proven false, or false because it
has not been proven true. For example, "Psychic phenomena don't
exist, since no-one has proved that they are real," or "Psychic phenomena
exist, since no-one has proven them to be false."
- Ad Hominem
- Literally, an argument directed "at the man". There are two forms
of ad hominem: the abusive variety occurs when
an arguer attacks those making an assertion, and the circumstantial
variety occurs when one argues that one's opponent ought to accept the
truth of an assertion because of the opponent's particular circumstances.
- Fallacies of Accident and Reverse Accident (Dicto Simpliciter)
- The Fallacy of Accident (Secundum Quid) is committed when a general rule is applied to a
particular case whose circumstances make the rule inapplicable.
It is the fallacy made when one goes from the general to the
specific. For example: "Over 50% of students in America don't
finish high school. You are a student in America. Therefore you
probably won't finish high school". Whether this statement is true or
not depends on the situation that the person being spoken to is in.
The fallacy of Reverse Accident (a.k.a. "Hasty Generalisation") occurs
when one goes from the specific to the general. For example:
"Jane said she didn't like me. Therefore no-one likes me."
- Begging the Question
- Begging the Question occurs when one "proves" a statement by
assuming that it's true to begin with. This can be done subtly.
In symbolic logic, begging the question can be denoted by the logical
statement X -> X, which doesn't really prove
anything logically. Because English is such a rich language, it's
easy to do this quite subtly. Consider a sentence that I was
going to write in an earlier draft of this page: "Begging the question is not a valid method of drawing conclusions, because you can't make deductions by assuming what you want to prove." If you read the previous sentence carefully, you'll note that both the premise and the conclusion are the same, so nothing
- Loaded Question
- A loaded question (also known as a complex question) is a question
that is comprised of two statements and so may contain hidden assumptions.
An example of such a question is "Have you stopped using drugs yet?".
Whether you answer either "yes" or "no" to this question, you imply that
you have used drugs.
- In English as well as other languages, many words have multiple
meanings. Changing the meaning of a word in mid-argument is committing
the fallacy of Equivocation. Here are some examples:
- Teaching people logic teaches them how to argue. Since people already
argue too much, there is no need to teach logic.
- If someone breaks the law, they are acting irresponsibly. Therefore,
anyone who breaks the law is not responsible for his actions and therefore
that person's actions aren't their fault.
- Scientific authorities assert that smoking causes cancer, but I know many
scientists who can't even control their own kids very well, so what kind of
authorities could they be?
- Post Hoc Ergo Propter Hoc
- This fallacy occurs when something is assumed to be the cause of an
event merely because it happened before an event. For example,
"99 percent of hard drug users started with cannabis. Therefore,
marijuana is a 'gateway drug'".
- Fallacies of Composition and Division
- The fallacy of composition is to conclude that a property shared by
all members of a set must apply to that set, or that a property shared
by all parts of something must apply to the whole. For example:
"Sodium is highly reactive, and Chlorine is highly poisonous.
Table salt is sodium chloride, a compound containing sodium and chlorine.
Therefore salt is highly reactive and poisonous."
The fallacy of division is the converse of the fallacy of composition, that
is to conclude that a property of something must apply to all its parts,
or that a property of a set must apply to all members of that set.
- Slippery Slope
- This argument states that, if one event occurs, other harmful events
will also, without giving any proof of a causal relationship. For
example, "If we legalize marijuana, then we would have to legalize
heroin and crack and we'll have a nation full of drug addicts on welfare.
Therefore we cannot afford to legalize marijuana".
- Bifurcation occurs when one assumes that only two alternatives
are available in a situation, when in truth there are others.
- Fallacy of the continuum
- The assumption that small differences are always unimportant. For example, "I know the budget is tight, but what difference will $1 a day make?"
- Illicit contrast
- This fallacy involves leading the listener to draw an inappropriate inference by placing unusual emphasis on certain words or phrases.
- Argument by innuendo
- This fallacy involves directing the listener to a certain conclusion by a careful selection of words or phrases. For example: "Is Joe a good person? Well, we've never caught him doing anything illegal."
- Red Herring
- To commit this fallacy is to introduce irrelevant material to the issue
being discussed, so that everyone's attention is diverted away from the
points being made, towards a different conclusion.
- Straw Man
- The straw man fallacy is to misrepresent a position so that it is
easier to "knock down", proceed to do so, and then claim that the
original position has been destroyed.
- Extended Analogy
- This fallacy is to assume that two different situations are analogous
to one another. Typically found when arguing about a general rule
that involves the two situations.
- Tu Quoque
- "Tu Quoque" means "you too" in Latin. This fallacy occurs when an
action is said to be acceptable because the opposing party has done it.
- Person 1
- You were driving way too fast there!
- Person 2
- So? You drive too quickly too.