Logical Fallacies
Logical and Factual Errors
Arguments consist of premises, inferences, and conclusions. Arguments containing bad inferences, i.e. inferences where the premises don't give adequate support for the conclusion drawn, can certainly be called fallacious. What is less clear is whether arguments containing false premises but which are otherwise fine should be called fallacious. If a fallacy is an error of reasoning, then strictly speaking such arguments are not fallacious; their reasoning, their logic, is sound. However, many of the traditional fallacies are of just this kind. It's therefore best to define fallacy in a way that includes them; this site will therefore use the word fallacy in a broad sense, including both formal and informal fallacies, and both logical and factual errors.
What is a Logical Fallacy?
A logical fallacy is, roughly speaking, an error of reasoning. When someone adopts a position, or tries to persuade someone else to adopt a position, based on a bad piece of reasoning, they commit a fallacy. I say "roughly speaking" because this definition has a few problems, the most important of which are outlined below. Some logical fallacies are more common than others, and so have been named and defined. When people speak of logical fallacies they often mean to refer to this collection of well-known errors of reasoning, rather than to fallacies in the broader, more technical sense given above.
Bandwagon Fallacy
Explanation The bandwagon fallacy is committed by arguments that appeal to the growing popularity of an idea as a reason for accepting it as true. They take the mere fact that an idea suddenly attracting adherents as a reason for us to join in with the trend and become adherents of the idea ourselves. This is a fallacy because there are many other features of ideas than truth that can lead to a rapid increase in popularity. Peer pressure, tangible benefits, or even mass stupidity could lead to a false idea being adopted by lots of people. A rise in the popularity of an idea, then, is no guarantee of its truth. The bandwagon fallacy is closely related to the appeal to popularity; the difference between the two is that the bandwagon fallacy places an emphasis on current fads and trends, on the growing support for an idea, whereas the appeal to popularity does not. Example (1) Increasingly, people are coming to believe that Eastern religions help us to get in touch with our true inner being. Therefore: (2) Eastern religions help us to get in touch with our true inner being. This argument commits the bandwagon fallacy because it appeals to the mere fact that an idea is fashionable as evidence that the idea is true. Mere trends in thought are not reliable guides to truth, though; the fact that Eastern religions are becoming more fashionable does not imply that they are true.
Ad Hominem (Personal Attack)
Explanation: It is important to note that the label "ad hominem" is ambiguous, and that not every kind of ad hominem argument is fallacious. In one sense, an ad hominem argument is an argument in which you offer premises that you the arguer don't accept, but which you know the listener does accept, in order to show that his position is incoherent (as in, for example, the Euthyphro dilemma). There is nothing wrong with this type of argument ad hominem. The other type of ad hominem argument is a form of genetic fallacy. Arguments of this kind focus not on the evidence for a view but on the character of the person advancing it; they seek to discredit positions by discrediting those who hold them. It is always important to attack arguments, rather than arguers, and this is where arguments that commit the ad hominem fallacy fall down. Example: (1) William Dembski argues that modern biology supports the idea that there is an intelligent designer who created life. (2) Dembski would say that because he's religious. Therefore: (3) Modern biology doesn't support intelligent design. This argument rejects the view that intelligent design is supported by modern science based on a remark about the person advancing the view, not by engaging with modern biology. It ignores the argument, focusing only on the arguer; it is therefore a fallacious argument ad hominem.
Fallacist's Fallacy
Explanation: The fallacist's fallacy involves rejecting an idea as false simply because the argument offered for it is fallacious. Having examined the case for a particular point of view, and found it wanting, it can be tempting to conclude that the point of view is false. This, however, would be to go beyond the evidence. It is possible to offer a fallacious argument for any proposition, including those that are true. One could argue that 2+2=4 on the basis of an appeal to authority: "Simon Singh says that 2+2=4″. Or one could argue that taking paracetamol relieves headaches using a post hoc: "I took the paracetamol and then my headache went away; it worked!" Each of these bad arguments has a true conclusion. A proposition therefore should not be dismissed because one argument offered in its favour is faulty. Example: "People argue that there must be an afterlife because they just can't accept that when we die that's it. This is an appeal to consequences; therefore there is no life after death."
Fallacy of Composition
Explanation: The fallacy of composition is the fallacy of inferring from the fact that every part of a whole has a given property that the whole also has that property. This pattern of argument is the reverse of that of the fallacy of division. It is not always fallacious, but we must be cautious in making inferences of this form. Examples: A clear case of the fallacy of composition is this: (1) Every song on the album lasts less than an hour. Therefore: (2) The album lasts less than an hour. Obviously, an album consisting of many short tracks may itself be very long. Not all arguments of this form are fallacious, however. Whether or not they are depends on what property is involved. Some properties, such as lasting less than an hour, may be possessed by every part of something but not by the thing itself. Others, such as being bigger than a bus, must be possessed by the whole if possessed by each part. One case where it is difficult to decide whether the fallacy of composition is committed concerns the cosmological argument for the existence of God. This argument takes the contingency of the universe (i.e. the alleged fact that the universe might not have come into being) as implying the existence of a God who brought it into being. The simplest way to argue for the contingency of the universe is to argue from the contingency of each of its parts, as follows: (1) Everything in the universe is contingent (i.e. could possibly have failed to exist). Therefore: (2) The universe as a whole is contingent (i.e. could possibly have failed to exist. It is clear that this argument has the form of the fallacy of composition; what is less clear is whether it really is fallacious. Must something composed of contingent parts itself be contingent? Or might it be that the universe is necessarily existent even though each of its parts is not? Another controversial example concerns materialistic explanations of consciousness. Is consciousness just electrical activity in the brain, as mind-brain identity theory suggests, or something more? Opponents of mind-brain identity theory sometimes argue as follows: (1) The brain is composed of unconscious neurons. Therefore: (2) The brain itself is not conscious. It is certainly difficult to see how consciousness can emerge from purely material processes, but the mere fact that each part of the brain is unconscious does not entail that the whole brain is the same.
Informal Fallacies
Inductive arguments needn't be as rigorous as deductive arguments in order to be good arguments. Good inductive arguments lend support to their conclusions, but even if their premises are true then that doesn't establish with 100% certainty that their conclusions are true. Even a good inductive argument with true premises might have a false conclusion; that the argument is a good one and that its premises are true only establishes that its conclusion is probably true. All inductive arguments, even good ones, are therefore deductively invalid, and so "fallacious" in the strictest sense. The premises of an inductive argument do not, and are not intended to, entail the truth of the argument's conclusion, and so even the best inductive argument falls short of deductive validity. Because all inductive arguments are technically invalid, different terminology is needed to distinguish good and bad inductive arguments than is used to distinguish good and bad deductive arguments (else every inductive argument would be given the bad label: "invalid"). The terms most often used to distinguish good and bad inductive arguments are "strong" and "weak". An example of a strong inductive argument would be: (1) Every day to date the law of gravity has held. Therefore: (2) The law of gravity will hold tomorrow. Arguments that fail to meet the standards required of inductive arguments commit fallacies in addition to formal fallacies. It is these "informal fallacies" that are most often described by guides to good thinking, and that are the primary concern of most critical thinking courses and of this site.
Taxonomy of Fallacies
Once it has been decided what is to count as a logical fallacy, the question remains as to how the various fallacies are to be categorised. The most common classification of fallacies groups fallacies of relevance, of ambiguity, and of presumption. Arguments that commit fallacies of relevance rely on premises that aren't relevant to the truth of the conclusion. The various irrelevant appeals are all fallacies of relevance, as are ad hominems. Arguments that commit fallacies of ambiguity, such as equivocation or the straw man fallacy, manipulate language in misleading ways. Arguments that commit fallacies of presumption contain false premises, and so fail to establish their conclusion. For example, arguments based on a false dilemma or circular arguments both commit fallacies of presumption. These categories have to be treated quite loosely. Some fallacies are difficult to place in any category; others belong in two or three. The 'No True Scotsman' fallacy, for example, could be classified either as a fallacy of ambiguity (an attempt to switch definitions of "Scotsman") or as a fallacy of presumption (it begs the question, reinterpreting the evidence to fit its conclusion rather than forming its conclusion on the basis of the evidence).
Formal Fallacies (Deductive Fallacies)
Philosophers distinguish between two types of argument: deductive and inductive. For each type of argument, there is a different understanding of what counts as a fallacy. Deductive arguments are supposed to be water-tight. For a deductive argument to be a good one (to be "valid") it must be absolutely impossible for both its premises to be true and its conclusion to be false. With a good deductive argument, that simply cannot happen; the truth of the premises entails the truth of the conclusion. The classic example of a deductively valid argument is: (1) All men are mortal. (2) Socrates is a man. Therefore: (3) Socrates is mortal. It is simply not possible that both (1) and (2) are true and (3) is false, so this argument is deductively valid. Any deductive argument that fails to meet this (very high) standard commits a logical error, and so, technically, is fallacious. This includes many arguments that we would usually accept as good arguments, arguments that make their conclusions highly probable, but not certain. Arguments of this kind, arguments that aren't deductively valid, are said to commit a "formal fallacy".
Formal and Informal Fallacies
There are several different ways in which fallacies may be categorised. It's possible, for instance, to distinguish between formal fallacies and informal fallacies.
