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. An error that stems from a poor logical form is sometimes called formal fallacy or simply an invalid argument.
- This article is about characteristics of fallacies, not about all things in life which could be considered fallacies
Before 20th century 
- FALLACY is a logical term; but in the consideration of the ideas denoted by it, we are led, at several points, beyond the ground occupied by Logic Proper. Especially there is often involved, in the scrutiny of fallacies, examination of the matter of arguments, that is, of the nature of the —objects argued about. The chapters on fallacies, therefore, which appear as appendices in the most elaborate logical treatises, really travel more or less out of the proper domain of the science; and the topic may here deserve a few paragraphs of separate illustration.
A fallacy is an unsound or inconclusive argument; an argument supposed or alleged to prove a conclusion which it does not prove. The name is sometimes confined to sophisms, that is, unsound arguments used with the intention to deceive. But the intention is a point of secondary importance in the theory of fallacies; and, indeed, those fallacies in which the reasoner deceives himself are by far more dangerous than the others, because they are by far more common. The term Fallacy, it will thus be observed, is applicable to an argument taken as a whole, not to any of the propositions of which the argument is composed. The propositions severally must be true or false: the argument which they constitute must be correct or fallacious ; that is, its conclusion must either follow or not follow from the premises.
- In the case of any person whose judgment is really deserving of confidence, how has it become so? Because he has kept his mind open to criticism of his opinions and conduct. Because it has been his practice to listen to all that could be said against him; to profit by as much of it as was just, and expound to himself, and upon occasion to others, the fallacy of what was fallacious. Because he has felt, that the only way in which a human being can make some approach to knowing the whole of a subject, is by hearing what can be said about it by persons of every variety of opinion, and studying all modes in which it can be looked at by every character of mind. No wise man ever acquired his wisdom in any mode but this; nor is it in the nature of human intellect to become wise in any other manner.
- All the fallacies of human reason had to be exhausted, before the light of a high truth could meet with ready acceptance.
- Max Müller (1860) History of Ancient Sanksrit Literature. p.32
- A Fallacy, or Sophism, is a false argument; or else an argument leading to a false conclusion. The use of such arguments is sometimes called sophistry; and in complex cases, it may be very difficult to detect. When the premises are false, or unsupported, or irrelevant, the fallacy is called material; but when the error is in the process of employing them, the fallacy is called logical.
- Roswell Park (1844) Pantology; or a systematic survey of human knowledge. p.80 §.4 (direct link)
- Fallacy: An argument which seems conclusive, but is not so, is fallacious ; that is to say, deceptive. When such fallacies are designedly framed, for the purpose of misleading those to whom they are addressed, they are called sophisms. A fallacy takes place - 1st, When some one of the things affirmed in an argument, and which is important to the conclusion, is not true. 2d, When the connexion between one fact and another is not truly stated. 3d, When the words implied are used in one sense, in one part of the argument, and in another, in another part : or when the sense assigned to such terms is so vague or ambiguous, that an error slips in, as it were, unperceived, in the course of the argument.
- The easiest and most popular way of practically refuting... any Fallacy is, by bringing forward a parallel case, where it leads to a manifest absurdity. A metaphysical objection may still be urged against many cases in which we thus reason from calculation of chances; an objection not likely indeed practically to influence any one, but which may afford the Sophist a triumph over those who are unable to find a solution.
- Richard Whately (1828) Elements of Rhetoric. p.52-53
20th century, first half 
- Fools! Do you argue, that things ancient ought, on that account, to be true and noble! Fallacies and Falsehoods there were from time immemorial, and dare you argue that because these are ancient these should prevail?
In ancient times, do you think that there was not the ignorant, and the shallow minded? And why after all should you embrace so fondly a carcass of dead thoughts. Live in the present and shape the future, do not be casting lingering looks to the distant past for the past has passed away, never again to return.
- Subramanya Bharathi (1882–1921) as quoted in "The People's Poet" by N. Nandhivarman in TamilSydney (7 January 2008)
- Fallacies do not cease to be fallacies because they become fashions.
- G. K. Chesterton Illustrated London News (19 April 1930)
- Utility and necessity of logic - It would be a mistake to imagine that, above and beyond what is called the Natural Logic of sound common sense, the study of the Science of Logic is absolutely necessary for right reasoning. Men reasoned rightly before Aristotle ever formulated a canon of logic. It was, in fact, by an analysis of such reasonings that he discovered those canons : they could never have been discovered otherwise. Here as elsewhere the art came before the science ; theory followed practice. A man may reason rightly without knowing a single rule of the syllogism ; or, conversely, he may know all the details of logic and be an indifferent guide to truth just as a first-rate geometrician may be a failure as an engineer. But still, just as his knowledge of geometry will enable the geometrician to detect the defects in a piece of engineering, so too will an explicit knowledge of the canons of reasoning enable us to discover more readily where the fallacy of a misleading argument lies. Without professing to guard us infallibly from error, logic familiarizes us with the rules and canons to which right reasoning processes must conform, and with the hidden fallacies and pitfalls to which such processes are commonly exposed.
- Peter Coffey (1912) The Science of Logic. Longmans, Green, and Company. p.38
- 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" falacies, by which he understood mistakes due to assuming false or unproven premisses, or premisses which prove the wrong conclusion.
- Peter Coffey (1918) The Science of Logic. 2e ed. Longmans, Green, and Company. p.302
- Fallacies of Assumption are those errors in reasoning which occur when the assumptions on which an argument rests are not clearly distinguished from the judgments of which the argument consists. An assumption, in this connection, is anything we take for granted, but do not assert, about the subject matter of an argument. It is the equivalent of what we… speak of as the universe of discourse. Interpreted from this point of view, it is readily seen that an assumption is not an assertion, and forms no part of the asserted contents of an argument, although, as we have seen, it has a relation to the argument, a relation which is indicated with sufficient clearness by saying that it points out the sphere of reference in which what is asserted may or not be accepted. Now it is a misinterpretation of the relation between what an argument assumes and what it asserts that lies at the foundation of the fallacies that we have here to consider. Thus, when what is taken for granted or assumed is allowed to function in any part of an argument as an assertion or judgment, or when the assumption on which an argument proceeds is ambiguous, the resulting fallacy is one of assumption.
Petitio Princpii is the name of an argument which assumes the conclusion that is to be proved… “the surreptitious assumption of a truth you are pretending to prove.” Since, then, the fallacy is one of assumption… its source must be found, not in what is definitely asserted, but in the world of reality or existence in which what is asserted has a definite meaning or fulfillment, that is to say, in the universe of discourse from the standpoint of which the argument is interpreted… Whenever it exists, the fallacy directs attention to the fact that the truth of what an argument asserts depends in part upon what assumptions the argument makes; and, in view of the nature of an argument, it follows that when assumptions are put forward as reasons we necessarily fail to establish a conclusion, and fall into the merest dogmatism unless we are willing to have these assumptions called into question. … Now, when this happens, when in the course of argument assumptions take the place of reasoned judgments, the argument is fallacious because, for the reason assigned, it involves a petitio principii… When the fallacy of petitio principii is committed in a single step it is called… hysteron proteron… and when it involves more than a single step it is called circulus in probando or reasoning in a circle.
Closely connected with the foregoing is the fallacy of the Complex Question. By a complex question, in the broadest meaning of that term, is meant one that suggests its own answer. Any question, for instance, that forces us to select, and assert in our answer to it, one of the elements of the question itself, while some other possibility is really open, is complex in the sense in which that term is here employed. If, for example, one were to ask… if your favourite colour were red or blue, or if you had given up a particular bad habit, he would be guilty of the fallacy of the complex question, if, in each case, the alternatives, as a matter of fact, were more numerous than, or were in any way different from, those stated in the question. Any leading question which complicates an issue by over simplification is fallacious for the same reason. Now, in the light of what we have said with respect to the petitio principii, it is not difficult to see that the fallacy of the complex question is occasioned by the character of the assumption on which the question rests. In the petitio principii an assumption with respect to the subject-matter of an argument functions as a premise, in the complex question it is a similar assumption that shuts out some of the material possibilities of a situation and confines an issue within too narrow limits. As in the former case, so here, the only way of meeting the difficulty is to raise the previous question, that is, to call the assumption which lies back of the fallacy into question.
Ignoratio Elenchi, according to Aristotle, is a fallacy which arises from “ignorance of the nature of refutation.” In order to refute an assertion, Aristotle says we must prove its contradictory; the proof, consequently, of a proposition which stood in any other relation than that to the original, would be an ignoratio elenchi… Since Aristotle, the scope of the fallacy has been extended to include all cases of proving the wrong point… “I am required to prove a certain conclusion; I prove, not that, but one which is likely to be mistaken for it; in that lies the fallacy… For instance, instead of proving that ‘this person has committed an atrocious fraud,’ you prove that ‘this fraud he is accused of is atrocious;’” … The nature of the fallacy, then, consists in substituting for a certain issue another which is more or less closely related to it, and arguing the substituted issue. The fallacy does not take into account whether the arguments do or do not really support the substituted issue, it only calls attention to the fact that they do not constitute a proof of the original one… It is a particularly prevalent and subtle fallacy and it assumes a great variety of forms. But whenever it occurs and whatever form it takes, it is brought about by an assumption that leads the person guilty of it to substitute for a definite subject of inquiry another which is in close relation with it. In the petitio principii the fallacy may be described as an assumption of the premises; in the complex question, as an assumption of the answer; and in the ignoratio elenchi, as an assumption of the question at issue.
- Arthur Ernest Davies (1915) A Text-Book of Logic. R.G. Adams and company. p.569-576
- It is not proof that I sought. I, of all men, know that proof is but a fallacy invented by man to justify to himself and his fellows his own crass lust and folly.
- William Faulkner (1933) Beyond. The Judge speaking.
20th century, second half 
- A common fallacy in much of the adverse criticism to which science is subjected today is that it claims certainty, infallibility and complete emotional objectivity. It would be more nearly true to say that it is based upon wonder, adventure and hope.
- Sir Cyril Norman Hinshelwood, as quoted in E.J. Bowen's obituary of Hinshelwood, Chemistry in Britain (1967), Vol. 3, 536
- It is just as easy to indoctrinate with fallacies as with facts.
- Aldo Leopold (1947) The Ecological Conscience, p.343
- We shouldn't decide everything by polling the masses. This is the fallacy argumentum ad numerum, the idea that something is true because great numbers believe it, as in "Eat shit. 20 trillion flies can't be wrong."
- The specialist is one who never makes small mistakes while moving towards the grand fallacy.
- One of the most prevalent fallacies is the so-called genetic fallacy, which tempts men to argue that the first lowly origins of a thing demonstrate what it essentially is even in its most highly developed forms. Psychoanalysis and anthropologists have sometimes specialized in tracing the golden fruits to their grubby roots, and they have had some success in convincing the credulous that greatness is only triviality writ large. A kindred fallacy—which to state is to expose—teaches that the surest way of understanding a type is to inspect its poorest instances.
- R. W. K. Paterson (1998) The New Patricians p. 2
- The general nature of the speech act fallacy can be stated as follows, using “good” as our example. Calling something good is characteristically praising or commending or recommending it, etc. But it is a fallacy to infer from this that the meaning of “good” is explained by saying it is used to perform the act of commendation.
- John Searle (1969) Speech Acts: An Essay in the Philosophy of Language. p.139
- The assertion fallacy ... is the fallacy of confusing the conditions for the performance of the speech act of assertion with the analysis of the meaning of particular words occurring in certain assertions.
- John Searle (1969) Speech Acts: An Essay in the Philosophy of Language. p.141
21st century 
- Fallacious arguments usually have the deceptive appearance of being good arguments.
- T. Edward Damer (2009) Attacking Faulty Reasoning: A Practical Guide to Fallacy-free Arguments 6th ed. Belmont, California: Wadsworth
- Never say, and never take seriously anyone who says, "I cannot believe that so-and-so could have evolved by gradual selection". I have dubbed this kind of fallacy "the Argument from Personal Incredulity". Time and again, it has proven the prelude to an intellectual banana-skin experience.
- Richard Dawkins (1995) River out of Eden
- The fact that man produces a concept "I" besides the totality of his mental and emotional experiences or perceptions does not prove that there must be any specific existence behind such a concept. We are succumbing to illusions produced by our self-created language, without reaching a better understanding of anything. Most of so-called philosophy is due to this kind of fallacy.
- Albert Einstein (1997) Einstein's God
- Since the time of Plato and Aristotle philosophers have had an interest in taking note of common fallacies in reasoning.
- Randal Marlin (2002) Propaganda & The Ethics Of Persuasion Chapter Three, Propaganda Technique, p. 110
- This fallacy [appeal to authority] is not in itself an error; it is impossible to learn much in today’s world without letting somebody else crunch the numbers and offer us explanations. And teachers are sources of necessary information. But how we choose our "authorities" and place a value on such information, is just another skill rarely taught in our education systems. It’s little wonder that to most folk, sound bites and talking heads are enough to count as experts. […] Teaching is reinforcing the appeal to authority, where anybody who seems more intelligent than you must ultimately be right. […] We educators must simply role-model critical thinking. […] Educators themselves have to be prepared to show that “evidence” and “answers” are two separate things by firmly believing that, themselves.
- Mike McRae, Australian teacher and guest columnist, "Educating Future Critical Thinkers", Swift: Online Newsletter of the JREF, 31 March 2006