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A fallacy is an argument that uses poor reasoning. An argument can be fallacious whether or not its conclusion is true. A fallacy can be either formal or informal. An error that stems from a poor logical form is sometimes called a formal fallacy or simply an invalid argument. An informal fallacy is an error in reasoning that does not originate in improper logical form. Arguments committing informal fallacies may be formally valid, but still fallacious.
There are many different informal fallacies, but a few basic types. For instance, material fallacies is error in what the arguer is talking about, while Verbal fallacies is error in how the arguer is talking.
Fallacies of presumption fail to prove the conclusion by assuming the conclusion in the proof. Fallacies of weak inference fail to prove the conclusion with insufficient evidence. Fallacies of distraction fail to prove the conclusion with irrelevant evidence, like emotion. Fallacies of ambiguity fail to prove the conclusion due to vagueness in words, phrases, or grammar.
Some fallacies are committed intentionally (to manipulate or persuade by deception), others unintentionally due to carelessness or ignorance.
Formal and material fallacies 
Modern logic textbooks use various systems to categorize fallacies.
Aristotle was the first to systematize logical errors into a list. Aristotle's "Sophistical Refutations" (De Sophisticis Elenchis) identifies thirteen fallacies. He dived them up into two major types, Verbal fallacies and Material fallacies. A Material Fallacy is an error in what the arguer is talking about, while a Verbal Fallacy is an error in how the arguer is talking. Verbal fallacies are those in which a conclusion is obtained by improper or ambiguous use of words.
Aristotle's distinction between fallacies in dictione and fallacies extra dictionem is not the same as [Richard] Whately's division into logical, and non-logical or material. By "logical" fallacies Whately meant those in which "the conclusion does not follow" from the premisses; by "material," those in which the "conclusion does follow" from the premisses. In the former class, the defect of proof lies either in a manifest violation of some of the formal laws of the syllogism--quaternio terminorum, undistributed middle, illicit major, illicit minor, negative premisses, etc., defects which remain even when symbols are substituted for the terms and concepts, and which Aristotle would not regard as sophisms owing to the transparency of the mistake;--or the defect lies in a similar violation masked in ambiguous language. The transparent defects Whately called purely logical, the cloaked defects semi-logical fallacies. The latter he regarded as all alike reducible to ambiguous middle term, including in this class all Aristotle's sophisms except the ignoratio elenchi, the petitio principii, and the non causa pro causa. These three he included in his "material" fallacies, by which he understood mistakes due to assuming false or unproven premisses, or premisses which prove the wrong conclusion. Whately's main distinction--between formally inconclusive arguments, and other sources of error--is sound and intelligible. But his nomenclature is objectionable. It is due to his narrow, nominalistic view of the scope of logic. All fallacies are logical, inasmuch as they are violations of logical principles or canons. Then, although most of Aristotle's sophismata, included in Whately's class of "semi-logical" fallacies, do in fact usually lead to formally invalid syllogisms through ambiguous middle terms, yet this is not clear in regard to some; and they certainly may lead to error otherwise as well. Hence the attempt to group them under such a head is unsatisfactory. Finally, on his own view of the scope of logic, Whately should not have dealt at all with what he called "non-logical" or "material" fallacies.
The distinction between a "formal" fallacy and a "material" fallacy is not fixed or clear--any more than that between "formal" and "material" logic. But at all events in a reasoning process, we can distinguish between the narrower "formal" or "consistency" aspect, which is independent of the truth of the premisses and the meaning of the terms used, and the "material" or "truth" aspect. Now, the formal validity of an inference, in this narrow sense, being independent of the subject-matter, i.e. of the meaning of the concepts and terms employed, it is only when the invalidity persists with the symbols, i.e. when some of the formal laws of reasoning are violated, that the fallacy is a formal one. If the fallacy lies in the language, .i.e. in the meaning of the terms employed, in ambiguitites of meaning, then its source is in the subject-matter, in the things for which the terms stand, and the fallacy is a material fallacy. An ambiguous middle term in a syllogism is, therefore, in this sense a material fallacy: when its two distinct meanings are explicitly substituted for it by two distinct terms, we have immediately the formal fallacy of quaternio terminorum. In this meaning of the expression "material fallacy," all Aristotle's sophismata in dictione are, when they enter into an inference, material fallacies; while some of his fallacies extra dictionem are formal in the sense that they can be represented in sysmbols; so that it is a mistake to confound Aristotle's two lists with Whately's semi-logical and material fallacies, respectively: a mistake into which Jevons seems to have fallen.—Peter Coffey, The Science of Logic
Common examples 
Intentional Fallacies 
Sometimes a speaker or writer uses a fallacy intentionally.
In humor, errors of reasoning are used for comical purposes. Groucho Marx used fallacies of amphiboly, for instance, to make ironic statements; Gary Larson employs fallacious reasoning in many of his cartoons. Wes Boyer and Samuel Stoddard have written a humorous essay teaching students how to be persuasive by means of a whole host of informal and formal fallacies.
In academic debate, in a conversation among friends, political discourse, or advertising, the arguer may use fallacious reasoning in order persuade the listener of the truth of the conclusion by any other means than offering relevant evidence.
For instance, the speaker or writer might divert the argument to unrelated issues using a red herring; insult someone's character (argumentum ad hominem), assume they are right by "begging the question" (petitio principi); make jumps in logic non-sequitur; identify a false cause and effect (post hoc ergo propter hoc); assert that everyone agrees, the bandwagoning; create a "false dilemma" or "either-or fallacy" in which the situation is oversimplified; selectively use facts or "card-stacking"; make false or misleading comparisons with "false equivalence", and "false analogy"; generalize quickly and sloppily with a "false generalization", and many more.
Deductive fallacy 
However, the same terms are used in informal discourse to mean an argument which is problematic for any reason.
A logical form such as A and B is independent of any particular conjunction of meaningful propositions. Logical form alone can guarantee that given true premises, a true conclusion must follow. However, formal logic makes no such guarantee if any premise is false; the conclusion can be either true or false. Any formal mistake or logical fallacy similarly invalidates the deductive guarantee. The so-called fallacy is a failure to understand that all bets are off unless the argument is formally flawless and all premises are true.
Paul Meehl's Fallacies 
- In Why I Do Not Attend Case Conferences (1973), psychologist Paul Meehl discusses several fallacies that can arise in case conferences that are primarily held to diagnose patients. These fallacies can also be considered more general errors of thinking that all individuals (not just psychologists) are prone to making.
- Barnum effect: Making a statement that is trivial, and true of everyone, or in case conferences of all patients, and is thus not of any use for discussion. Everyone will agree on it, but it will not provide any incremental help in making predictions.
- sick-sick fallacy ("pathological set"): The tendency to have our own stereotypes of what is "healthy", based on our own experiences and ways of being, and identifying others who are different than ourselves as "sick." Meehl emphasizes that though psychologists claim to know about this tendency, most are not very good at correcting it in their own thinking.
- "me too" fallacy: The opposite of sick-sick , thinking that "anybody would do this." Minimizing a symptom without considering the objective probability that a mentally healthy person would experience it. Is this really a "normal" characteristic?
- Uncle George's pancake fallacy: A variation of "me too", this refers to minimizing a symptom by calling to mind a friend/relative who exhibited a similar symptom, thereby implying that it is normal and common. Meehl points out that the proper conclusion in this comparison is not that the patient is healthy by comparison, but that your friend/relative is unhealthy by comparison.
- Multiple Napoleon's fallacy: "It's not real to us, but it's 'real' to him." A theoretical turn that Meehl sees as an unnecessary waste of time. There is a distinction between reality and delusion, and it is important to make this distinction when assessing the patient. Pondering the patient's reality can be misleading and distracting from the importance of their delusion in making a diagnostic decision.
- hidden decisions: Meehl identifies the decisions we make about patients that we do not explicitly own up to and do not often challenge. For example, placing middle and upper class patients in long term therapy, while lower class patients are more likely to be medicated. This is related to the implicit ideal patient- young, attractive, verbal, intelligent, and successful (termed YAVIS)- that we would much rather have in psychotherapy, in part because they can pay for it long term and in part because they are potentially more enjoyable to interact with.
- the spun-glass theory of the mind: The belief that the human organism is so fragile that minor negative events, such as criticism, rejection, or failure, are bound to cause major trauma to the system. Essentially not giving humans, and sometimes patients, enough credit when it comes to their resiliency and ability to recover.
Other systems of classification 
Of other classifications of fallacies in general the most famous are those of Francis Bacon and J. S. Mill. Bacon (Novum Organum, Aph. 33, 38 sqq.) divided fallacies into four Idola (Idols, i.e. False Appearances), which summarize the various kinds of mistakes to which the human intellect is prone. With these should be compared the Offendicula of Roger Bacon, contained in the Opus maius, pt. i. J. S. Mill discussed the subject in book v. of his Logic, and Jeremy Bentham's Book of Fallacies (1824) contains valuable remarks. See Rd. Whateley's Logic, bk. v.; A. de Morgan, Formal Logic (1847) ; A. Sidgwick, Fallacies (1883) and other textbooks.
See also 
- Association fallacy
- Cognitive bias
- Cognitive distortion
- Fallacies of definition
- False premise
- False statement
- Invalid proof
- Mathematical fallacy
- Victim blaming
- Whig history
- Hurley, Patrick J. (2005). A Concise Introduction to Logic. Wadsworth. p. 656. ISBN 0534585051.
- Coffey, P. (1912). The Science of Logic. Longmans, Green, and Company. p. 302. LCCN 12018756.
- Boyer, Web. "How to Be Persuasive". Retrieved 12/05/2012.
- Ed Shewan (2003). Applications of Grammar: Principles of Effective Communication (2nd ed.). Christian Liberty Press. pp. 92 ff. ISBN 1-930367-28-7.
- Meehl, P.E. (1973). Psychodiagnosis: Selected papers. Minneapolis (MN): University of Minnesota Press, p. 225-302.
- Fearnside, W. Ward and William B. Holther, Fallacy: The Counterfeit of Argument, 1959.
- Vincent F. Hendricks, Thought 2 Talk: A Crash Course in Reflection and Expression, New York: Automatic Press / VIP, 2005, ISBN 87-991013-7-8
- D. H. Fischer, Historians' Fallacies: Toward a Logic of Historical Thought, Harper Torchbooks, 1970.
- Warburton Nigel, Thinking from A to Z, Routledge 1998.
- T. Edward Damer. Attacking Faulty Reasoning, 5th Edition, Wadsworth, 2005. ISBN 0-534-60516-8
- Sagan, Carl, "The Demon-Haunted World: Science As a Candle in the Dark". Ballantine Books, March 1997 ISBN 0-345-40946-9, 480 pgs. 1996 hardback edition: Random House, ISBN 0-394-53512-X, xv+457 pages plus addenda insert (some printings). Ch.12.
Further reading 
- C. L. Hamblin, Fallacies, Methuen London, 1970. reprinted by Vale Press in 1998 as ISBN 0-916475-24-7.
- Douglas N. Walton, Informal logic: A handbook for critical argumentation. Cambridge University Press, 1989.
- Hans V. Hansen; Robert C. Pinto (1995). Fallacies: classical and contemporary readings. Penn State Press. ISBN 978-0-271-01417-3.
- John Woods (2004). The death of argument: fallacies in agent based reasoning. Springer. ISBN 978-1-4020-2663-8.
- Frans van Eemeren; Bart Garssen; Bert Meuffels (2009). Fallacies and Judgments of Reasonableness: Empirical Research Concerning the Pragma-Dialectical Discussion. Springer. ISBN 978-90-481-2613-2.
- Aristotle, On Sophistical Refutations, De Sophistici Elenchi. library.adelaide.edu.au
- William of Ockham, Summa of Logic (ca. 1323) Part III.4.
- John Buridan, Summulae de dialectica Book VII.
- Francis Bacon, the doctrine of the idols in Novum Organum Scientiarum, Aphorisms concerning The Interpretation of Nature and the Kingdom of Man, XXIIIff. fly.hiwaay.net
- Arthur Schopenhauer, The Art of Controversy | Die Kunst, Recht zu behalten - The Art Of Controversy (bilingual), (also known as "Schopenhauers 38 stratagems"). gutenberg.net
- John Stuart Mill, A System of Logic - Raciocinative and Inductive. Book 5, Chapter 7, Fallacies of Confusion. la.utexas.edu
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- Fallacy entry in the Internet Encyclopedia of Philosophy
- Infographic poster of common logical fallacies
- Appeal to Authority Appeal to Authority Logical Fallacy
- FallacyFiles.org contains categorization of fallacies with examples.
- 42 informal logical fallacies explained by Dr. Michael C. Labossiere (including examples), nizkor.org
- Humbug! The skeptic’s field guide to spotting fallacies in thinking – textbook on fallacies. scribd.com
- List of fallacies with clear examples, infidels.org
- Interactive Syllogistic Machine A web based syllogistic machine for exploring fallacies, figures, and modes of syllogisms.
- Logical Fallacies and the Art of Debate, csun.edu
- An Informal Fallacy Primer, acontrario.org
- Stephen Downes Guide to the Logical Fallacies, onegoodmove.org
- "Love is a Fallacy", a short story written by Max Shulman.
- WebCitation archive.