Informal fallacy  

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Kunstformen der Natur (1904) by Ernst Haeckel
Kunstformen der Natur (1904) by Ernst Haeckel

In contrast to a formal fallacy, an informal fallacy originates in a reasoning error other than a flaw in the logical form of the argument. A deductive argument containing an informal fallacy may be formally valid, but still remain rationally unpersuasive. Nevertheless, informal fallacies apply to both deductive and non-deductive arguments.

Though the form of the argument may be relevant, fallacies of this type are the "types of mistakes in reasoning that arise from the mishandling of the content of the propositions constituting the argument".


Faulty generalization

A special subclass of the informal fallacies is the set of faulty generalizations, also known as inductive fallacies. Here the most important issue concerns inductive strength or methodology (for example, statistical inference). In the absence of sufficient evidence, drawing conclusions based on induction is unwarranted and fallacious. With the backing of empirical evidence, however, the conclusions may become warranted and convincing (at which point the arguments are no longer considered fallacious).

Hasty generalization

For instance, hasty generalization is making assumptions about a whole group or range of cases based on a sample that is inadequate (usually because it is atypical or just too small). Stereotypes about people ("frat boys are drunkards", "grad students are nerdy", "women don’t enjoy sports", etc.) are a common example of the principle.

Hasty generalisation often follows a pattern such as:

X is true for A.
X is true for B.
Therefore, X is true for C, D, etc.

While never a valid logical deduction, if such an inference can be made on statistical grounds, it may nonetheless be convincing. This is because with enough empirical evidence, the generalization is no longer a hasty one.

Relevance fallacy

The fallacies of relevance are a broad class of informal fallacies (see the navbox below), generically represented by missing the point: presenting an argument, which may be sound, but fails to address the issue in question.

Argumentum ex silentio

An argument from silence features an unwarranted conclusion advanced based on the absence of data.

Examples of informal fallacies

Post hoc (false cause)

This fallacy gets its name from the Latin phrase "post hoc, ergo propter hoc," which translates as "after this, therefore because of this." Definition: Assuming that because B comes after A, A caused B. Sometimes one event really does cause another one that comes later—for example, if I register for a class, and my name later appears on the roll, it's true that the first event caused the one that came later. But sometimes two events that seem related in time aren't really related as cause and event. That is, temporal correlation doesn't necessarily entail causation.

Slippery slope

Definition: The arguer claims that a sort of chain reaction, usually ending in some dire consequence, will take place, but in fact there is not enough evidence for that assumption. The arguer asserts that if we take even one step onto the "slippery slope," we will end up sliding all the way to the bottom; they assume we can't stop halfway down the hill.

False analogy

This error in reasoning occurs when claims are supported by unsound comparisons, hence the false analogy's informal nickname of the "apples and oranges" fallacy.

Unless indicated otherwise, the text in this article is either based on Wikipedia article "Informal fallacy" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on original research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

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