Logical Fallacies

Logic is the study of reasoning -- the nature of good (correct) reasoning and of bad (incorrect) reasoning. Its focus is the method by which an argument unfolds, not whether any arbitrary statement is true or accurate. Thus, an argument can be both deductively valid and perfectly absurd, as in 1. All telephone poles are elephants. 2. Sally is a telephone pole. 3. Therefore, Sally is an elephant. The conclusion is valid because it conforms to a correct syllogistic pattern -- in this case, affirmation of the antecedent -- but is ludicrous at the same time.

As a "branch" of philosophy, logic is often broken down into many subsets: for instance, modal logic, many-valued logic, modern logic, symbolic logic, formal and informal logic, deductive and inductive logic. Those interested in pursuing the subject in depth should read and carefully examine the long articles in The Encyclopedia of Philosophy, especially "A Glossary of Logical Terms." Each article is followed by an extensive bibliography. (See also Logic.)

A fallacy is an invalid form of argument, an instance of incorrect reasoning. Below is a list of common fallacies. Hit the "Back" button to return to the top.

List of Fallacies:

affirming the consequent

anthrocentric (human-centered) fallacy

appeal to authority

a priori fallacies

arguing from "is" to "ought"

argumentum ad baculinum

argumentum ad captandum

argumentum ad crumenam

argumentum ad hominem

argumentum ad ignorantiam

argumentum ad lazarum

argumentum ad misericordiam

argumentum ad populum

argumentum ad verecundiam (see "appeal to authority")

argumentum ex silentio

begging the question

circular reasoning


fallacy of false alternatives

fallacies of interrogation


gadarene swine fallacy

genetic fallacy

hasty generalization

if-then fallacies

ignoratio elenchi

invincible ignorance

naturalistic fallacy

non sequitur


performative contradiction

petitio principii (see "begging the question")

poisoning the wells

post hoc ergo propter hoc

red herring

straw man fallacy

tu quoque fallacy

undistributed middle

Further Investigation

affirming the consequent -- A fallacy of the form "if A, then B; B, therefore A". Example: "If Smith testifies against Jones in court, Jones will be found guilty. Jones was found guilty. Therefore, Smith must have testified against him." {Jones could have been found guilty without Smith's testimony.}

anthrocentric (human-centered) fallacy -- This one isn't found in standard texts, but was described by John Stuart Mill in System of Logic. Consider the example of a preacher who one day takes someone supposedly possessed of a demon, throws his hand on her forehead, and shouts, "Get out! Leave this body!" Even supposing that demons exist, one might find it curious that they understand English, obey peremptory commands, and are easily influenced by incantations and rituals. The a.f. here occurs at the presupposition level: human language, reason, instincts, and desires are assumed to be the orbit around which everything else in the universe (including the aforementioned demons) revolve.

appeal to authority -- Known also as the argumentum ad verecundiam fallacy. An appeal to authority is ordinarily one good way to buttress a line of thought. The practice becomes fallacious when one of the following happens: the authority is not an expert in the field in which one is speaking; the allusion to authority masks the fact that experts may be divided down the middle on the subject; no explicit reference is made to the authority.

a priori fallacies -- From The Encyclopedia of Philosophy: "Under the heading of a priori fallacies Mill listed a number of natural prejudices, including the popular superstition that words have a magical power and such philosophical dogmas as that which is true of our ideas of things must be true of the things themselves; that differences in nature must correspond to our received (linguistic) distinctions; that whatever is, is rationally explicable; that there is no action at a distance; that every phenomenon has a single cause; and that effects must resemble their causes. These are all errors, but we can go further and recognize a general apriorist fallacy, which consists in trying to base knowledge of fundamental synthetic truths on anything other than empirical evidence.

arguing from "is" to "ought" -- A fallacy first articulated by David Hume (1711-1776) in which someone argues from a premise containing only a descriptive term, to a conclusion containing an "ought." Example: "There is nothing morally wrong with the institution of slavery. It has been with us in some form for thousands of years." (The fact that slavery has been with us or is with us is not moral justification of the act. What is may not be the same thing as what ought to be.)

argumentum ad baculinum -- Fallacy that occurs when threat of force is made, either implicitly or explicitly. Example: "I'm willing to discuss this in even more depth, but if you don't come around soon, there may be dire consequences." (Baculum from the Latin means "stick".)

argumentum ad captandum -- Any specious or unsound argument that is likely to win popular acceptance. (literally, "for catching the common herd").

argumentum ad crumenam -- The fallacy of supposing that a conclusion must be valid because the person making the argument is wealthy. (Crumena from the Latin means "purse".) An instance of this fallacy is when someone turns to another and says, "Well, if you're so smart, why aren't you rich?" One can be both smart and poor, as indeed numerous philosophers throughout history were (e.g., Lao-Tzu, Socrates, Spinoza).

argumentum ad hominem ("argument against the person") -- A common fallacy in which someone argues against a position or claim by assailing the proponent of it. The truth or falsehood of a position doesn't depend on who does (or doesn't) espouse it. e.g., "You can't trust Jones' theory of electromagnetic particles because he's a communist." (The theory is good or bad because it comports (or doesn't comport) with certain facts and evidence, not because the man propounding it holds a political affiliation.)

argumentum ad ignorantiam ("arguing from ignorance") -- A fallacy that occurs when someone argues that because we don't know something is true, it must be false, or because we lack proof that a statement is false, it must be true. Ignorance or lack of evidence doesn't necessarily mean a position or claim is true or false. Common Examples: "No one has ever proven that UFOs exist. Therefore, they don't exist." (Something can exist despite the absence of confirmation. Lack of proof is justification for caution or even scepticism, but not dogmatic assertions.) "There is simply no proof that God exists. Therefore, God doesn't exist." (God might exist even though there is no way empirically to prove it.)

argumentum ad lazarum -- The fallacy of supposing a conclusion is valid because the argument is made by a poor person. It is the opposite of the ad crumenam fallacy.

argumentum ad misericordiam -- Occurs when an appeal is made to pity or to one's sympathetic nature. Example: "Augusto Pinochet is an old, dying man. It is wrong to make him stand trial for alleged offenses."

argumentum ad populum -- This fallacy occurs when an argument panders to popular passion or sentiment. When, for instance, a politician exclaims in a debate that his opponent "is out of step with the beliefs of everyone in the audience," he/she is committing the fallacy. The legitimacy of a statement depends not on its popularity, but on its truth credentials.

argumentum ex silentio -- The fallacy of supposing that someone's silence is necessarily proof of ignorance. Two people, for instance, may be debating a political issue on a cable news program. One may be in the studio with the host, the other appearing via satellite. Their time on air reaches the point when each only has a few seconds left to make a closing comment. One of the debaters asks his opponent a very technical, complex question, and the opponent is speechless for a few seconds. "Go ahead," the debater screams. "Answer my question! See? He can't answer." A viewer may be left with the impression that the person's speechlessness is tantamount to ignorance, when in fact any number of things could have happened: 1) the satellite connection could've been lost or experiencing problems; 2) the debater was thinking about how best to answer a difficult question under such an immediate time constraint; 3) the debater might not have even heard the whole question. There may be reasons for temporary silence other than ignorance.

begging the question -- Circular reasoning in which a claim is assumed to be true and is then tucked in the conclusion. e.g., "Government by the people is ideal because democracy is the least inadequate form of government." ("Government by the people" is the working definition of democracy; the first part of the statement needs to be proven, not reasserted in the predicate.)

circular reasoning -- Sometimes known as circulus in demonstrando, or begging the question. H.W. Fowler, in Modern English Usage, puts it this way: "The basing of two conclusions each upon the other. That the world is good follows from the known goodness of God; that God is good is known from the excellence of the world he has made."

equivocation -- Sometimes referred to as "amphiboly". A fallacy that stems from the ambiguous meaning of certain words. For example, 1. Only man is logical. 2. No woman is a man. 3. Therefore, no woman is logical. "Man" in the first sentence really means "mankind," "humankind," "homo sapiens". "Man" in the second sentence means "maleness". The syllogism appears to be valid, but in fact is fallacious because of the subtle shift in meaning.

fallacy of false alternatives -- A fallacy occurring when the number of alternatives is said to be fewer than the actual number. Common examples of this fallacy are statements containing either/or, nothing/but, all-or-nothing elements. Examples: "Is she a Democrat or a Republican?" (She may be a socialist, a libertarian, a Leninist, an anarchist, a feminist or any number of other things, including one who is strictly apolitical.) "If you aren't for your country, then you are against it." (One may be neither "for" nor "against" but may occupy a position of strict neutrality or be affirmative sometimes and critical at others.)

fallacies of interrogation -- There are two forms of this particular fallacy. One is asking two or more questions and demanding a single answer when, in fact, each question might require separate treatment. The other form is asking a question whose answer would necessitate acceptance of a presupposition, one which the answerer might separately deny. The famous example of this second form is asking, "Do you still beat your wife?" Answering "no" legitimates the question and does nothing to contradict the presupposition that the husband once did beat his wife. Asking a question with presuppositions is fine so long as a narrow answer is not demanded.

flamboyance -- The manner in which someone speaks can easily draw unwarranted support for a thesis or idea. Incisive wit, verbal facility, equanimity and repartee have no bearing at all on the soundness/legitimacy of a position. It is the essence of what is said, not the manner in which it is said, that counts. As Bertrand Russell once noted, the purpose of being educated is to defend ourselves against the seductions of eloquence.

genetic fallacy -- A fallacy that occurs when someone attacks the cause or origin of a belief rather than its substance. Why a person believes something is not relevant to the belief's legitimacy/soundness/validity. Example: "Smith's belief in God stems from a subsconscious need for a fatherly figure and is thus a total joke." (The psychological link may in fact be true and may even shed some light on the personality of Smith, but is nevertheless irrelevant to the truth/falsehood of his belief.)

hasty generalization -- The habit of arriving at a bold conclusion based on a limited sample of evidence. This often occurs with statistics. For instance, someone may ask ten women and one man what their opinion is of contemporary male-female relationships and from this sample draw a sweeping conclusion; hasty generalization would then be said to exist.

if-then fallacies -- 1. Affirming the consequent (If P, then Q. Q. Therefore P.). 2. Denying the antecedent (If P, then Q. Not P. Therefore not Q.) 3. Converting a conditional (If P, then Q. Therefore if Q, then P.) 4. Negating antecedent and consequent (If P, then Q. Therefore if not P, then not Q.)

ignoratio elenchi ("ignoring of the disproof") -- A fallacy that consists in disproving or proving something different from what is in question or called for. It can also be called the irrelevance fallacy. Example: "You cannot convict my client of murder. We have proven that one of the arresting officers made prejudicial remarks, remarks scornful of my client. Look at the videotape, the audiotape, the man's own testimony. He is a full-blown racist; you must not trust anything he says." (Undermining an allegation of murder is something different than proving that one member of the plaintiff's team is bigoted; hence, the i.e. fallacy here.)

invincible ignorance -- the fallacy of insisting on the legitimacy of one's position in the face of contradictory facts. Statements like "I really don't care what the experts say; no one is going to convince me that I'm wrong"; "nothing you say is going to change my mind"; "yeah, okay, whatever!" are examples of this fallacy.

naturalistic fallacy -- From the Encyclopedia of Philosophy: "What G.E. Moore called the naturalistic fallacy is the identifying of goodness with any natural characteristic, such as pleasantness or being the object of desire. If there is a distinct property, goodness, it will of course be an error to identify it with any other feature, even if the two are coextensive, and this would be an example of the refusal to distinguish what we cannot separate; however, it must first be shown that there is such a property as Moore's goodness. Alternatively, if it is a question of how the word 'good' is commonly used, then it would be an error to say that it is used to convey some natural description. However, if the naturalist is not trying to report the ordinary use, but is saying that this ordinary use is somehow unsatisfactory (and also that there is no such property as the one of which Moore speaks) and is therefore proposing a different use, where is his mistake? It is true that if he redefines 'good' as the name of some natural characteristic, but still also uses the word in its ordinary evaluative or prescriptive sense, he will be slipping into a fallacy of ambiguity; but a consistent ethical naturalist may be committing no fallacy at all."

non sequitur ("it does not follow") -- A statement that does not logically follow from what preceded it; a conclusion that does not follow from the premises.

paralogism -- Any fallacious or illogical argument generally.

poisoning the wells -- This entry comes from an article by Albury Castell titled "Analyzing A Fallacy," which was included in the book Readings In Speech, edited by Haig Bosmajian (Harper & Row, 1965). Here is the full quote: "During the last century a famous controversy took place between Charles Kingsley and Cardinal Newman. It began, I believe, by Kingsley suggesting that truth did not possess the highest value for a Roman Catholic priest; that some things were prized above truth. Newman protested that such a remark made it impossible for an opponent to state his case. How could Newman prove to Kingsley that he did have more regard for truth than for anything else, if Kingsley argued from the premiss that he did not? It is not merely a question of two persons entertaining contradictory opinions. It is subtler than that. To put it baldly, Newman would be logically 'hamstrung.' Any argument he might use to prove that he did entertain a high regard for truth was automatically ruled out by Kingsley's hypothesis that he did not. Newman coined the expression poisoning the wells for such unfair tactics...The phrase poisoning the wells exactly hits off the difficulty. If the well is poisoned, no water drawn from it can be used. If a case is so stated that contrary evidence is automatically precluded, no arguments against it can be used."

post hoc, ergo propter hoc ("after this, therefore because of this") -- This might also be described as the causality fallacy: Event Y follows from Event X, so one automatically concludes that X caused Y. (A young man walks by a neighbor's house and sees a cat scurrying away; he looks up and sees a giant hole in the window. The hole, he infers, must have been caused by the cat, who fell through the pane. The inference is hasty, because the hole might have been caused by any number of things -- a baseball that missed a friend's glove and flew over his head; young brothers fighting inside and accidentally smashing the window, etc.).

red herring -- An attempt to divert attention from the crux of an argument by introduction of anecdote, irrelevant detail, subsidiary facts, tangential references, and the like.

straw man -- A fallacy that occurs when someone attacks a less defensible position than the one actually being put forth. This occurs very often in politics, when one seeks to derive maximum approval for himself/herself or for a cause. Example: "Opposition to the North American Free Trade Agreement amounts to nothing but opposition to free trade." (Someone can believe in free and open trade and yet still oppose NAFTA.)

tu quoque ("you too") fallacy -- The fallacy of assuming an argument is specious because it is either inconsistent with the person's actions or inconsistent with previous claims/arguments. A person may "preach" about something and act in a very different manner, but this fact has no bearing on the specific argument he is advancing at any time. Inconsistency, moreover, may raise issues of hypocrisy or double standards, but it does not bear upon the argument at hand. Example: "Smith: If someone hits you, you should turn the other cheek. Violence only begets violence, and violence in and of itself is wrong. Jones: That's a joke. You used to hit people when they picked a fight with you." (Smith may not have practiced what he now preaches, but two of his premises -- that violence only begets violence, and that violence is wrong -- need to be carefully examined.)

undistributed middle -- A fallacy of the form "All A are B. All C are B. Therefore, all A are C." Consider: All elms are trees. All oaks are trees. Therefore, all elms are oaks.

Further Investigation:

Years of intense study and training are not needed to develop a rough understanding of logic or, for that matter, of any other branch of philosophy. Much ground can be gained by reading a few chapters of several books, by foraging through various collections in used or old bookstores, and by visiting a few good sites on the web.

Below is a list of prominent logicians and their work:

Aristotle: Categories; On Interpretation; Prior Analytics; Posterior Analytics; Topics; Sophistical Refutations.

Francis Bacon: Novum Organum

Descartes: Discourse on Method

John Dewey: Reconstruction In Philosophy

John Stuart Mill: System of Logic

W.V. Quine: Mathematical Logic.

Bertrand Russell: The Problems of Philosophy; The Principles of Mathematics.

Gilbert Ryle: Dilemmas

Ludwig Wittgenstein: Philosophical Investigations

Other Works:

Rudolf Carnap: Introduction to Symbolic Logic and its Applications.

Alonzo Church: Introduction to Mathematical Logic.

M.R. Cohen & Ernest Nagel: An Introduction to Logic and Scientific Method.

W.W. Fearnside & W.B. Holther: Fallacy -- The Counterfeit of Argument.