Fallacies of Ignorance by klimerp02


									The argument from ignorance, also known as argumentum ad ignorantiam ("appeal to ignorance" [1]), argument by lack of imagination, or negative evidence, is a logical fallacy in which it is claimed that a premise is true only because it has not been proven false, or is false only because it has not been proven true. The argument from personal incredulity, also known as argument from personal belief or argument from personal conviction, refers to an assertion that because one personally finds a premise unlikely or unbelievable, the premise can be assumed to be false, or alternatively that another preferred but unproven premise is true instead. Both arguments commonly share this structure: a person regards the lack of evidence for one view as constituting proof that another view is true. The types of fallacies discussed in this article should not be confused with the reductio ad absurdum method of argument, in which a valid logical contradiction of the form "A and not A" is used to disprove a premise. Overview Commonly in an argument from personal incredulity or argument from ignorance, the speaker considers or asserts that something is false, implausible, or not obvious to them personally and attempts to use this gap in knowledge as "evidence" in favor of an alternative view of his or her choice. Examples of these fallacies are often found in statements of opinion which begin: "It is hard to see how...," "I cannot understand how...," or "it is obvious that..." (if "obvious" is being used to introduce a conclusion rather than specific evidence in support of a particular view). [edit] Argument from ignorance The two most common forms of the argument from ignorance, both fallacious, can be reduced to the following form:

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Something is currently unexplained or insufficiently understood or explained, so it is not (or must not be) true. Because there appears to be a lack of evidence for one hypothesis, another chosen hypothesis is therefore considered proven.

[edit] Argument from personal incredulity Two common versions of the argument from personal incredulity are:

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"I can't believe this is possible, so it can't be true." (The person is asserting that a proposition must be wrong because he or she is [or claims to be] unable or unwilling to fully consider that it might be true, or is unwilling to believe evidence which does not support her or his preferred view.) "That's not what people say about this; people instead agree with what I am saying." (Here the person is asserting that a proposition must be inaccurate because the opinion of "people in general" is claimed to agree with the speaker's opinion, without offering specific evidence in support of the alternative view.) This is also called argumentum ad populum.

An argument from personal incredulity is the same as an argument from ignorance only if the person making the argument has solely their particular personal belief in the impossibility of the one scenario as "evidence" that the alternative scenario is true (i.e., the person lacks relevant evidence specifically for the alternative scenario). Quite commonly, the argument from personal incredulity is used in combination with some evidence in an attempt to sway opinion towards a preferred conclusion. Here too, it is a logical fallacy to the degree that the personal incredulity is offered as further "evidence." In such an instance, the person making the argument has inserted a personal bias in an attempt to strengthen the argument for acceptance of her or his preferred conclusion. (Also see similar arguments: wisdom of repugnance and argument from emotion) [edit] Negative evidence A related concept is negative evidence. This is commonly problematic in cladistics, such as the classification of organisms or languages. If two members of a group share a feature and a third does not, the two may be classified together as a subgroup excluding the third. However, without evidence that the trait was an innovation (apomorphy) in the two, rather than a loss of an

ancestral feature (plesiomorphy) in the third, the lack of a feature is not a logically valid argument for such a classification. Two possible explanations of the situation are as follows: common acquisition origin of feature C common origin A B loss of feature C A B

For example, in the classification of the Kradai languages of southern China and Southeast Asia, there are four clear branches: the Tai, Kra, Kam–Sui, and Hlai languages (excluding for argument's sake the problematic Be language). Traditionally the Kam– Sui and Tai branches were classified together, due to the large amount of vocabulary that they share. It was argued that this vocabulary was an innovation that defined a genealogical clade within the family. However, this amounts to a lack of evidence of this vocabulary in the other branches; another possibility is that it is common Kradai vocabulary that was lost in Kra and Hlai. Indeed, morphological evidence suggests that Tai is closer to Hlai, and Kam–Sui is closer to Kra, than Tai and Kam–Sui are to each other, pace the negative evidence from vocabulary. Phenetics, a now largely superseded method similar to cladistics, is particularly prone to this kind of error. As opposed to cladistics, where the relationships are resolved via evaluation of the evolutionary history of features, in phenetics plesiomorphies and apomorphies are treated identically. Thus, a phenetic hypothesis about relationships rests on the quantity (not quality) of evidence alone. In a cladistic analysis of Kradai languages, the stronger (though quantitatively less) evidence from morphology can reveal the plentiful evidence from vocabulary to be a conserved plesiomorphy, while a phenetic analysis would be overwhelmed by the sheer amount of vocabulary evidence. [edit] Burden of proof An important aspect of the ad ignorantiam argument is establishing the burden of proof. While this concept is discussed in the law section of this page, it is important to realize that establishing the burden of proof is important in other arenas as well. All logic follows from presuppositions (axiomatic statements, see axiom). These presuppositions are not provable but are assumed as true. [edit] Inductive usage Inductive usage refers to the extension of an argument to support a wider generalization of a hypothesis, principle, scientific theory, or universal law. Many such uses of the Argument from Ignorance are considered fallacious, especially in academic papers which are expected to be rigorous about their key premises and empirical foundations. However, in some cases (such as that which the noted author Irving Copi describes below) where affirmative evidence could reasonably be expected to be found, but following careful unbiased examination, this evidence has still not been found, then it might become expedient, and sometimes even prudent, to infer that this might suggest (though it does not prove, deductively, it suggests inductively) that the evidence does not exist. Or, where the speaker can reasonably assume that all sane people will agree with a premise (e.g. "The sky is blue"), then he might decide it is unnecessary to provide evidence supporting that assertion; however, these issues (to which epistemological foundationalism is closely related, and with which it is also closely intertwined) are still debated. [edit] Description Irving Copi writes that: The argumentum ad ignorantiam [fallacy] is committed whenever it is argued that a proposition is true simply on the basis that it has not been proven false, or that it is false because it has not been proven true. He adds, A qualification should be made at this point. In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence despite searching, as positive evidence towards its non-occurrence. (Copi 1953)

To support this, one might add a third case, the argument that something is false or true because the speaker cannot (or finds it hard to) conceive otherwise. This argument by lack of imagination is sometimes expressed in the form "Y is absurd (because I cannot imagine it), therefore it must be untrue," or "It is hard to see how..." [ie I personally cannot see, or lack the imagination to see, how], and is sometimes confused with the logically valid method of argument, reductio ad absurdum. A logical argument using reductio ad absurdum would be framed as "X logically leads to a provably impossible (absurd) conclusion, therefore it must be false." In reductio ad absurdum, it is necessary to show that accepting X implies a contradiction (such as "not X", or "Y and not Y" for some other proposition Y). In an argument from ignorance, the speaker asserts "X implies not Y", where Y is believed to be, but cannot be proven, true, rather than something which is provably contradictory. Copi's argument concerns the Y condition; that in this case of "X implies not Y" for some other proposition Y, some weight must be given to the probability that the speaker's evaluation of Y is correct. For example, if proposition X is "This man was shot", and proposition Y is "There was no bullet", the speaker's qualification to assert condition Y must be considered. A coroner who had examined the body is most likely qualified to draw this conclusion, but an eyewitness is probably unqualified. Argument from personal incredulity is very similar, e.g. "I am unable to believe/understand X, therefore it must be false." [edit] Examples


"If polar bears are (the) dominant (predator) in the Arctic, then there would seem to have been no need for them to evolve a white-coloured form of camouflage." In his book Probability of God, Anglican Bishop Hugh Montefiore casts doubt on neo-Darwinian evolution with that statement. This argument was addressed by the evolutionary theorist Richard Dawkins in his book The Blind Watchmaker, who wrote that if the writer had thought to imagine a black polar bear trying to sneak up on a seal in the Arctic, he would see the evolutionary value of such fur. The ignorance in this case was assuming that no other purpose could be served.

[edit] Law In most modern criminal legal systems there is a presumption of innocence, and it is the responsibility of the prosecution to prove (usually "beyond reasonable doubt") that a defendant has in fact committed a particular crime. It is a logical fallacy to presume that mere lack of evidence of innocence of a crime is instead evidence of guilt. Similarly, mere lack of evidence of guilt cannot be taken as evidence of innocence. For this reason (among others), Western legal systems err on the side of caution. Simply the act of taking a defendant before a court is not adequate evidence to presume anything. Courts require evidence of guilt to be presented first, adequate for the court to find that the charge has been substantiated -- i.e., that the prosecution's evidentiary burden has been met -- and only after this burden is met is the defense obliged to present counterevidence of innocence. If the burden of proof is not met, that does not imply that the defendant is innocent. Hence, in such a case, the defendant is found "not guilty", except in Scotland, where the jury also has the option to return a verdict of not proven. Also, as a hypothetical example of an "argument from personal incredulity," defined above, suppose someone were to argue:

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I cannot imagine any way for Person P to have executed action X without committing a crime Y Therefore, Person P must be guilty of crime Y.

Merely because the person making the argument cannot imagine how scenario "A" might have happened does not necessarily mean that the person's preferred conclusion (scenario "B") is correct. As with other forms of the argument from ignorance, the arguer in this instance has arrived at a conclusion without any evidence supporting the preferred hypothesis, merely for lack of being able to imagine the alternative. The same principles of logic apply to the civil law, although the required burdens of proof generally are different. As well, these principles of logic apply to the introduction of a given component of a legal case by either a complainant or a defendant. That is, the mere lack of evidence in favor of a proposition put forth by a party in a legal proceeding (e.g., the assertion "she couldn't have left the house and returned in time to do X..." is offered without evidence in support) would not properly be taken as evidence in favor of an alternative explanation (e.g., "she did leave the house and return in time to do X..."). [edit] Science Unexplained phenomena are an indication that a particular scientific theory does not provide a satisfactory model sufficient to explain or predict all outcomes. For example, the wave theory of light does not explain the photoelectric effect, though it

successfully predicts the results of the double-slit experiment. However, later theories based around quantum mechanics provide an adequate explanatory model of both. It is a logical mistake to assert that because a phenomenon is unpredictable by current scientific theories, that a better scientific theory cannot be found that provides an adequate natural explanatory model for the phenomena in question; and that therefore, one must assert that the only viable explanation of the unexplained phenomena is the supernatural action of God. This variant is known as the God-of-the-gaps argument. However, it is also logically incorrect to assume that because a theory does explain all known relevant phenomena, it must be correct. The fact that no counter-examples are known to exist is not in itself proof of a given theory, since there is always the possibility of some yet-to-be-observed counter-example. For example, there are no known phenomena that are inconsistent with the Big Bang theory. This by no means constitutes definitive proof that the universe actually did originate with the Big Bang.

Argument from authority or appeal to authority is a logical fallacy, where it is argued that a statement is correct because the statement is made by a person or source that is commonly regarded as authoritative. The most general structure of this argument is: Source A says that p. Source A is authoritative. Therefore, p is true. This is a fallacy because the truth or falsity of the claim is not necessarily related to the personal qualities of the claimant, and because the premises can be true, and the conclusion false (an authoritative claim can turn out to be false). It is also known as argumentum ad verecundiam (Latin: argument to respect) or ipse dixit (Latin: he himself said it). [1] On the other hand, arguments from authority are an important part of informal logic. Since we cannot have expert knowledge of many subjects, we often rely on the judgments of those who do. There is no fallacy involved in simply arguing that the assertion made by an authority is true, the fallacy only arises when it is claimed or implied that the authority is infallible in principle and can hence be exempted from criticism: It can be true, the truth can merely not be proven, or made probable by attributing it to the authority, and the assumption that the assertion was true might be subject to criticism and turn out to have actually been wrong. If a criticism appears that contradicts the authority's statement, then merely the fact that the statement originated from the authority is not an argument for ignoring the criticism. Forms There are two basic forms of appeal to authority, based on the authority being trusted. The more relevant the expertise of an authority, the more compelling the argument. Nonetheless, authority is never absolute, so all appeals to authority which assert that the authority is necessarily infallible are fallacious. The first form of the appeal to authority is when a source presenting a position on a subject mentions some authority who also holds that position, but who is not actually an authority in that area. For instance, the statement "Arthur C. Clarke released a report showing it is necessary to floss three times daily" should not convince many people of anything about flossing, as Clarke, a science fiction writer, was not a known expert on dental care. Much advertising relies on this logical fallacy in the form of endorsements and sponsorships. A sportsperson or actor, for example, is no more likely than average to have an specialist knowledge of watches or perfume, but their endorsement of a particular brand of watch or perfume is very valuable in advertising terms. Alternatively they may not be experts in the relevant part of the field (for example, an expert in litigation may not be an expert on trust law or commercial law even though they are indeed a civil lawyer). In some cases, the advertisers use an actor's well-known role to imply that the person has authority in an area; an actor who plays a doctor on television may appear in their white coat, and endorse a drug or health product. The second form, citing a source who is actually an authority in the relevant field, carries more subjective, cognitive weight. A person who is recognized as an expert authority often has greater experience and knowledge of their field than the average person, so their opinion is more likely than average to be correct. In practical subjects such as car repair, an experienced mechanic who knows how to fix a certain car will be trusted to a greater degree than someone who is not an expert in car repair. There are many cases where one must rely on an expert, and cannot be reasonably expected to have the same experience, knowledge and skill that that person has. Many trust a surgeon without ever needing to know all the details about surgery

themselves. Nevertheless, experts can still be mistaken, wilfully deceptive, subject to pressure from peers or employers, or have unusual views (or views that are widely criticized by other experts) within their field, and hence their expertise does not always guarantee that their arguments are valid. In some cases, the appeal to authority plays on the Western culture's respect for credentials. For example, suppose a complex nutritional system and diet guide is endorsed or ghostwritten and credited to a qualified doctor. While a doctor does receive general training on nutrition and diet, they may not be an expert on nutrition and diet, a field for which an expert will often possess PhDs in nutrition and certification as a dietician. The same technique is used with the PhD degree; an advertiser may reinforce their claims about a product by appending an endorsement from John Doe, PhD, but without stating what area the PhD is in. If the product being endorsed is foot powder, and Dr. Doe studied podiatry, the endorsement carries some weight, but if he studied film criticism, he may have no more than average knowledge of the product and its merits. In mathematics, the second form, especially when the appellant is himself the authority, is wryly referred to as "proof by tenure". [edit] Appeal to authority as logical fallacy A (fallacious) appeal to authority argument has the basic form: 1. 2. 3. A makes claim B; there is something positive about A that (fallaciously) is used to imply that A has above-average or expert knowledge in the field, or has an above-average authority to determine the truth or rightness of such a matter therefore claim B is true, or has its credibility unduly enhanced as a result of the proximity and association.

The first statement is called a 'factual claim' and is the pivot point of much debate. The last statement is referred to as an 'inferential claim' and represents the reasoning process. There are two types of inferential claim, explicit and implicit. The converse, that (fallaciously) relies on something negative about the source and claims that therefore the conclusion is probably false, is called an ad hominem argument. [edit] Examples of appeals to authority [edit] Arguments

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Referring to the philosophical beliefs of Aristotle: "If Aristotle said it was so, it is so." Referring to the philosophical beliefs of Jesus, Muhammad, or any other religious figure: "If (religious figure) said it was so, it is so." Such an appeal may be based upon the belief that the speaker in question is holy and, by extension, inerrant. Alternately, the figure may be considered to be an expert on the given subject: "Buddha was a great moral teacher and he said that euthanasia is wrong, so it must be wrong." Referring to a sacred text: "If (the text) said it was so, it is so." Like in the previous example, such an appeal may be based upon the belief that the sacred text in question is inerrant. This argument may also present a false dilemma situation, where the text can be interpreted in multiple dissimilar ways. Referring to a famous text or work: "Democracy in America criticized American political party division, so we ought to promote bipartisanship." Quoting a well-known personage: "As Samuel Johnson said, patriotism is the last refuge of the scoundrel." Implying that, therefore, patriotism is always bad. (The term "patriot" was used at the time by radical followers of John Wilkes, whom the conservative Johnson opposed); or "There is no need to critically examine Plan A because [person's name] is in favour of it, and [person's name] is [experienced, knowledgeable, respected] in this field." Referring to what one is told by one's teacher and/or parent: "My teacher said so, therefore it must be so." Believing something because it is attributed to an honored profession, as in: "This doctor recommends (brand-name) aspirin" or "Bankers recommend that people have six months' wages in a savings account". Appealing to some reference or citation from a famous book or author without considering the actual truth of the citation. References in no way ensure, without any doubt, that the claim is true. References simply show where the information or claim possibly originated and to avoid plagiarism. Appeals to various well known opinion poll firms that are assumed to have collected the best data from a large enough sample, and that there were no leading questions.

[edit] The nature of the fallacy

An appeal to authority cannot guarantee the truth of the conclusion, given the nature of truth and the Consensus theory of truth, because the fact that an authority says something does not necessarily make it so. The fact that, objectively, a proposition is in fact true or that it has good unrelated arguments supporting it will be what makes authorities believe it to be true. The fallacy comes in when the opposite situation occurs, with authority opinions leading to the belief itself. Thus, an appeal to authority confuses cause and effect. As with all logical fallacies, the fact that an argument is an appeal to authority does not make its conclusion untrue (this line of thought is sometimes known as the logical fallacy fallacy) and does not make it unreasonable to believe the truth of the argument. It also must be noted that a rigorous concept of truth is a complex subject. In informal logic, the fact that a majority of experts in a given field believe X- for example, the fact that nearly all medical scientists think that HIV causes AIDS and reject AIDS denialism- makes it more reasonable for a person without knowledge in the field to believe X. The bandwagon fallacy is very similar to the appeal to authority, given that it- with popular opinion being cited in support of an idea rather than popular opinion coming to believe an idea based on the ideas own inherent truth—confuses cause and effect in the same way. In normal conversation, these two fallacies frequently intermingle. For example, consider the statement: "Basically everyone, economic experts included, supports the financial bailout and so must I." [edit] Epistemology without appeal to authority A philosophy which denies and rejects harshly the existence of any authority, proof, disproof, or justification, even only with probability, and holds everything open to criticism, including observation (that is, it even rejects the inference "X was observed directly → X is necessarily true" as an appeal to authority), logics and its own very basic positions, such as criticism itself, is pancritical rationalism. Without the need ever to appeal to authority for justification, the pancritical rationalist is able to hold his position with complete integrity, since he is not guilty of relativism or dogmatism.

An ad hominem argument, also known as argumentum ad hominem (Latin: "argument to the man", "argument against the man") consists of replying to an argument or factual claim by attacking or appealing to a characteristic or belief of the person making the argument or claim, rather than by addressing the substance of the argument or producing evidence against the claim. The process of proving or disproving the claim is thereby subverted, and the argumentum ad hominem works to change the subject. Background Ad hominem argument is most commonly used to refer specifically to the ad hominem abusive, or argumentum ad personam, which consists of criticizing or attacking the person who proposed the argument (personal attack) in an attempt to discredit the argument. It is also used when an opponent is unable to find fault with an argument, yet for various reasons, the opponent disagrees with it. Other common subtypes of the ad hominem include the ad hominem circumstantial, or ad hominem circumstantiae, an attack which is directed at the circumstances or situation of the arguer; and the ad hominem tu quoque, which objects to an argument by characterizing the arguer as acting or arguing in accordance with the view that he is arguing against. Ad hominem arguments are always invalid in syllogistic logic, since the truth value of premises is taken as given, and the validity of a logical inference is independent of the source making the inference. However, ad hominem arguments are rarely presented as formal syllogisms, and their assessment lies in the domain of informal logic and the theory of evidence.[1] The theory of evidence depends to a large degree on assessments of the credibility of witnesses, including eyewitness evidence and expert witness evidence. Evidence that a purported eyewitness is unreliable, or has a motive for lying, or that a purported expert witness lacks the claimed expertise can play a major role in making judgements from evidence. Argumentum ad hominem is the inverse of argumentum ad verecundiam, in which the arguer bases the truth value of an assertion on the authority of the source asserting it. Hence, while an ad hominem argument may make an assertion less compelling, by showing that the source making the assertion does not have the authority it claims, or has made mistaken assertions on similar topics in the past, it cannot provide an infallible counterargument. An ad hominem fallacy is a genetic fallacy and red herring, and is most often (but not always) an appeal to emotion.

It does not include arguments posed by a source that contradict the source's actions. [edit] Ad hominem as informal fallacy A (fallacious) ad hominem argument has the basic form: Person A makes claim X There is something objectionable about Person A Therefore claim X is false Ad hominem is one of the best known of the logical and systematic fallacies usually enumerated in introductory logic and critical thinking textbooks. Both the fallacy itself, and accusations of having committed it, are often brandished in actual discourse (see also Argument from fallacy). As a technique of rhetoric, it is powerful and used often because of the natural inclination of the human brain to recognize patterns.

The first premise is called a 'factual claim' and is the pivot point of much debate. The contention is referred to as an 'inferential claim' and represents the reasoning process. There are two types of inferential claim, explicit and implicit. The fallacy does not represent a valid form of reasoning because even if you accept both co-premises, that does not guarantee the truthfulness of the contention. This can also be thought of as the argument having an un-stated co-premise.

In this example, the un-stated co-premise "everything that A claims is false" has been included, and the argument is therefore now a valid one. However in the ad hominem fallacy the un-stated co-premise is always false, thereby maintaining the fallacy. Note that this does not imply that the contention "eugenics is a bad idea" is false, merely un-supported by the pattern of reasoning below it.

Usage In logic An ad hominem fallacy consists of asserting that an argument is wrong and/or the source is wrong to argue at all purely because of something discreditable/not-authoritative about the source or those sources cited by it rather than addressing the soundness of the argument itself. The implication is that the source's argument and/or ability to argue correctly lacks authority. Merely insulting a source in the middle of otherwise rational discourse does not necessarily constitute an ad hominem fallacy (though it is not usually regarded as acceptable). It must be clear that the purpose of the characterization is to discredit the source offering the argument, and, specifically, to invite others to discount its arguments. In the past, the term ad hominem was sometimes used more literally, to describe an argument that was based on an individual, or to describe any personal attack. However, this is not how the meaning of the term is typically introduced in modern logic and rhetoric textbooks, and logicians and rhetoricians are in agreement that this use is incorrect.[2] Example: "You claim that this man is innocent, but you cannot be trusted since you are a criminal as well." This argument would generally be accepted as reasonable, as regards personal evidence, on the premise that criminals are likely to lie to protect each other. On the other hand, it is a valid example of ad hominem if the source making the claim is doing so on the basis of evidence independent of its own credibility. In general, ad hominem criticism of evidence cannot prove the negative of the proposition being claimed: Example: "Paula says the umpire made the correct call, but this can't be true, because Paula is a stupid idiot." Assuming the premise is correct, Paula's evidence is valueless, but the umpire may nonetheless have made the right call. Colloquially In common language, any personal attack, regardless of whether it is part of an argument, is often referred to as ad hominem.[3] Types of ad hominems Three traditionally identified varieties are ad hominem abusive (or ad personam), ad hominem circumstantial, and ad hominem tu quoque. Ad hominem abusive Ad hominem abusive (also called argumentum ad personam) usually and most notoriously involves insulting or belittling one's opponent, but can also involve pointing out factual but ostensibly damning character flaws or actions which are irrelevant to the opponent's argument. This tactic is logically fallacious because insults and even true negative facts about the opponent's personal character have nothing to do with the logical merits of the opponent's arguments or assertions. This tactic is frequently employed as a propaganda tool among politicians who are attempting to influence the voter base in their favor through an appeal to emotion rather than by logical means, especially when their own position is logically weaker than their opponent's. Examples:

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"You can't believe Jack when he says God exists because he doesn't even have a job." "Candidate Jane Jones's proposal X is ridiculous. She was caught cheating on her taxes in 2003."

Ad hominem circumstantial Ad hominem circumstantial involves pointing out that someone is in circumstances such that he is disposed to take a particular position. Essentially, ad hominem circumstantial constitutes an attack on the bias of a source. The reason that this is fallacious in syllogistic logic is that pointing out that one's opponent is disposed to make a certain argument does not make the argument, from a logical point of view, any less credible; this overlaps with the genetic fallacy (an argument that a claim is incorrect due to its source). On the other hand, where the source taking a position seeks to convince us by a claim of authority, or personal observation, observation of their circumstances may reduce the evidentiary weight of the claims, sometimes to zero. [4] Examples:

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"Tobacco company representatives should not be believed when they say smoking doesn't seriously affect your health, because they're just defending their own multi-million-dollar financial interests." "He's physically addicted to nicotine. Of course he defends smoking!” "What do you know about politics? You're too young to vote!"

Mandy Rice-Davies's famous testimony, during the Profumo Affair, "Well, he would [say that], wouldn't he?", is an example of a valid circumstantial argument. Her point is that since a man in a prominent position, accused of an affair with a callgirl, would deny the claim whether it was true or false, his denial, in itself, carries little evidential weight against the claim of an affair. Note, however, that this argument is valid only insofar as it devalues the denial; it does not bolster the original claim. To construe evidentiary invalidation of the denial as evidentiary validation of the original claim is fallacious (on several different bases, including that of argumentum ad hominem); however likely the man in question would be to deny an affair that did in fact happen, he could only be more likely to deny an affair that never did. Ad hominem tu quoque Main article: tu quoque Ad hominem tu quoque (lit: "You too!") refers to a claim that the source making the argument has spoken or acted in a way inconsistent with the argument. In particular, if Source A criticizes the actions of Source B, a tu quoque response is that Source A has acted in the same way. Examples:

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"You say that stealing is wrong, but you do it as well." "He says we shouldn't enslave people, yet he himself owns slaves"

Guilt by association Main article: Association fallacy Guilt by association can sometimes also be a type of ad hominem fallacy, if the argument attacks a source because of the similarity between the views of someone making an argument and other proponents of the argument. This form of the argument is as follows: Source A makes claim P. Group B also make claim P. Therefore, source A is a member of group B.

Example: "You say the gap between the rich and poor is unacceptable, but communists also say this, therefore you are a communist" This fallacy can also take another form: Source A makes claim P. Group B make claims P and Q Therefore, Source A makes claim Q. Examples: "You say the gap between the rich and poor is unacceptable, but communists also say this, and they believe in revolution. Thus, you believe in revolution." A similar tactic may be employed to encourage someone to renounce an opinion, or force them to choose between renouncing an opinion or admitting membership in a group. For example: "You say the gap between the rich and poor is unacceptable. You don't really mean that, do you? Communists say the same thing. You're not a communist, are you?" Guilt by association may be combined with ad hominem abusive. For example: "You say the gap between the rich and poor is unacceptable, but communists also say this, and therefore you are a communist. Communists are unlikeable, and therefore everything they say is false, and therefore everything you say is false." A reductio ad Hitlerum argument can be seen as an example of a "guilt by association" fallacy, since it attacks a viewpoint simply because it was supposedly espoused by Adolf Hitler, as if it is impossible that such a man could have held any viewpoint that is correct. [edit] Inverse ad hominem An inverse ad hominem argument praises a source in order to add support for that source's argument or claim. A fallacious inverse ad hominem argument may go something like this: "That man was smartly-dressed and charming, so I'll accept his argument that I should vote for him" As with regular ad hominem arguments, not all cases of inverse ad hominem are fallacious. Consider the following: "Elizabeth has never told a lie in her entire life, and she says she saw him take the bag. She must be telling the truth." Here the arguer is not suggesting we accept Elizabeth's argument, but her testimony. Her being an honest person is relevant to the truth of the conclusion (that he took the bag), just as her having bad eyesight (a regular case of ad hominem) would give reason not to believe her. However, the last part of the argument is false even if the premise is true, since having never told a lie before does not mean she isn't now. Appeal to authority is a type of inverse ad hominem argument.

To be continued

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