an error of reasoning or tactic of argument that can be used to persuade someone with whom you are reasoning that your argument is correct when really it is not. The standard treatment of the informal fallacies in logic textbooks draws heavily on Aristotle’s list, but there are many variants, and new fallacies have often been added, some of which have gained strong footholds in the textbooks. The word ‘informal’ indicates that these fallacies are not simply localized faults or failures in the given propositions (premises and conclusion) of an argument to conform to a standard of semantic correctness (like that of deductive logic), but are misuses of the argument in relation to a context of reasoning or type of dialogue that an arguer is supposed to be engaged in. Informal logic is the subfield of logical inquiry that deals with these fallacies. Typically, informal fallacies have a pragmatic (practical) aspect relating to how an argument is being used, and also a dialectical aspect, pertaining to a context of dialogue – normally an exchange between two participants in a discussion. Both aspects are major concerns of informal logic. Logic textbooks classify informal fallacies in various ways, but no clear and widely accepted system of classification has yet become established. Some textbooks are very inventive and prolific, citing many different fallacies, including novel and exotic ones. Others are more conservative, sticking with the twenty or so mainly featured in or derived from Aristotle’s original treatment, with a few widely accepted additions. The paragraphs below cover most of these ‘major’ or widely featured fallacies, the ones most likely to be encountered by name in the language of everyday educated conversation. The genetic fallacy is the error of drawing an inappropriate conclusion about the goodness or badness of some property of a thing from the goodness or badness of some property of the origin of that thing. For example, ‘This medication was derived from a plant that is poisonous; therefore, even though my physician advises me to take it, I conclude that it would be very bad for me if I took it.’ The error is inappropriately arguing from the origin of the medication to the conclusion that it must be poisonous in any form or situation. The genetic fallacy is often construed very broadly making it coextensive with the personal attack type of argument (see the description of argumentum ad hominem below) that condemns a prior argument by condemning its source or proponent. Argumentum ad populum (argument to the people) is a kind of argument that uses appeal to popular sentiments to support a conclusion. Sometimes called ‘appeal to the gallery’ or ‘appeal to popular pieties’ or even ‘mob appeal,’ this kind of argument has traditionally been portrayed as fallacious. However, there need be nothing wrong with appealing to popular sentiments in argument, so long as their evidential value is not exaggerated. Even so, such a tactic can be fallacious when the attempt to arouse mass enthusiasms is used as a substitute to cover for a failure to bring forward the kind of evidence that is properly required to support one’s conclusion.
Argumentum ad misericordiam (argument to pity) is a kind of argument that uses an appeal to pity, sympathy, or compassion to support its conclusion. Such arguments can have a legitimate place in some discussions – e.g., in appeals for charitable donations. But they can also put emotional pressure on a respondent in argument to try to cover up a weak case. For example, a student who does not have a legitimate reason for a late assignment might argue that if he doesn’t get a high grade, his disappointed mother might have a heart attack.
The fallacy of composition is the error of arguing from a property of parts of a whole to a property of the whole – e.g., ‘The important parts of this machine are light; therefore this machine is light.’ But a property of the parts cannot always be transferred to the whole. In some cases, examples of the fallacy of composition are arguments from all the parts to a whole, e.g. ‘Everybody in the country pays her debts. Therefore the country pays its debts.’ The fallacy of division is the converse of that of composition: the error of arguing from a property of the whole to a property of its parts – e.g., ‘This machine is heavy; therefore all the parts of this machine are heavy.’ The problem is that the property possessed by the whole need not transfer to the parts.
The fallacy of false cause, sometimes called post hoc, ergo propter hoc (after this, therefore because of this), is the error of arguing that because two events are correlated with one another, especially when they vary together, the one is the cause of the other. For example, there might be a genuine correlation between the stork population in certain areas of Europe and the human birth rate. But it would be an error to conclude, on that basis alone, that the presence of storks causes babies to be born. In general, however, correlation is good, if sometimes weak, evidence for causation. The problem comes in when the evidential strength of the correlation is exaggerated as causal evidence. The apparent connection could just be coincidence, or due to other factors that have not been taken into account, e.g., some third factor that causes both the events that are correlated with each other.
The fallacy of secundum quid (neglecting qualifications) occurs where someone is arguing from a general rule to a particular case, or vice versa. One version of it is arguing from a general rule while overlooking or suppressing legitimate exceptions. This kind of error has also often been called the fallacy of accident. An example would be the argument ‘Everyone has the right to freedom of speech; therefore it is my right to shout ‘Fire’ in this crowded theater if I want to.’ The other version of secundum quid, sometimes also called the fallacy of converse accident, or the fallacy of hasty generalization, is the error of trying to argue from a particular case to a general rule that does not properly fit that case. An example would be the argument ‘Tweetie [an ostrich] is a bird that does not fly; therefore birds do not fly’. The fault is the failure to recognize or acknowledge that Tweetie is not a typical bird with respect to flying. Argumentum consensus gentium (argument from the consensus of the nations) is a kind that appeals to the common consent of mankind to support a conclusion. Numerous philosophers and theologians in the past have appealed to this kind of argument to support conclusions like the existence of God and the binding character of moral principles. For example, ‘Belief in God is practically universal among human beings past and present; therefore there is a practical weight of presumption in favor of the truth of the proposition that God exists’. A version of the consensus gentium argument represented by this example has sometimes been put forward in logic textbooks as an instance of the argumentum ad populum (described above) called the argument from popularity: ‘Everybody believes (accepts) P as true; therefore P is true’. If interpreted as applicable in all cases, the argument from popularity is not generally sound, and may be regarded as a fallacy. However, if regarded as a presumptive inference that only applies in some cases, and as subject to withdrawal where evidence to the contrary exists, it can sometimes be regarded as a weak but plausible argument, useful to serve as a provisional guide to prudent action or reasoned commitment. Argumentum ad hominem (literally, argument against the man) is a kind of argument that uses a personal attack against an arguer to refute her argument. In the abusive or personal variant, the character of the arguer (especially character for veracity) is attacked; e.g., ‘You can’t believe what Smith says – he is a liar’. In evaluating testimony (e.g., in legal cross-examination), attacking an arguer’s character can be legitimate in some cases. Also in political debate, character can be a legitimate issue. However, ad hominem arguments are commonly used fallaciously in attacking an opponent unfairly – e.g., where the attack is not merited, or where it is used to distract an audience from more relevant lines of argument. In the circumstantial variant, an arguer’s personal circumstances are claimed to be in conflict with his argument, implying that the arguer is either confused or insincere; e.g., ‘You don’t practice what you preach’. For example, a politician who has once advocated not raising taxes may be accused of ‘flip-flopping’ if he himself subsequently favors legislation to raise taxes. This type of argument is not inherently fallacious, but it can go badly wrong, or be used in a fallacious way, for example if circumstances changed, or if the alleged conflict was less serious than the attacker claimed. Another variant is the ‘poisoning the well’ type of ad hominem argument, where an arguer is said to have shown no regard for the truth, the implication being that nothing he says henceforth can ever be trusted as reliable.
Yet another variant of the ad hominem argument often cited in logic textbooks is the tu quoque (you-too reply), where the arguer attacked by an ad hominem argument turns around and says, ‘What about you? Haven’t you ever lied before? You’re just as bad.’ Still another variant is the bias type of ad hominem argument, where one party in an argument charges the other with not being honest or impartial or with having hidden motivations or