This post explains what a logical fallacy is, and lists a few of the common logical fallacies. In arguing points on sciforums, members should try to avoid committing any of these fallacies, as they all make for weak and flawed arguments. [size=+1]Logical Fallacies[/size] In formal terms, an argument in favour of a proposition consists of one or more premises and a conclusion. Premises are statements that are offered in support of the conclusion. A deductive argument is one in which the premises directly lead to the conclusion. An inductive argument is one in which the premises provide some support, or evidence, for the conclusion, but do not establish the conclusion beyond doubt. If all the premises in a deductive argument are true, then if the argument is good (i.e. there are no logical fallacies) the conclusion must be true. If one or more of the premises is false then the argument is unsound. If all the premises of an inductive argument are true, then the conclusion is likely, to a greater or lesser extent, to be true. A fallacy is a form of argument in which the conclusion does not follow from the premises for one reason or another. A logical fallacy is an error in reasoning, as opposed to an error about the facts. A logical fallacy in a deductive argument may appear as a set of true premises that do not imply the conclusion. A logical fallacy in an inductive argument is less formal - the given premises simply do not provide enough support for the conclusion. In that case, even if all the premises were true, the conclusion would still not be more likely to be true than it was before the argument was made. There are many logical fallacies. A taxonomy of fallacies can be found at the www.fallacyfiles.org, where fallacies are classified into groups and subgroups and the subgrouped fallacies share the overall features of their parent groups. I present here a selected list of some common logical fallacies, with examples. [size=+1]Formal fallacies[/size] Affirming the consequent Any argument of the form: if A is true then B is true. B is true. Therefore A is true. "If the Earth is flat then nothing will fall off the Earth. Nothing is observed to fall off the Earth. Therefore, the Earth is flat." Denying the antecedent Any argument of the form: if A is true then B is true. A is not true. Therefore, B is not true. "If you could walk all the way around the Earth in a straight line and end up where you started, then the Earth is round. You can't walk around the Earth. Therefore, the Earth is not round." [size=+1]Fallacies of relevance[/size] Fallacies of relevance attempt to support an argument by offering considerations that simply have no bearing on the truth of the matter at hand. Ad hominem An attempt to counter a claim by attacking the person making the claim rather than the substance of the claim itself. "You're an idiot! Therefore, the Earth is flat." "You only say the Earth is round because you have a vested interest in saying that." Tu quoque / Two wrongs make a right Literally, "you also". Attempt to justify wrong action or argument on the basis that somebody else also does or says the wrong thing. "My evidence for a flat Earth may be faked, but some of the people who argue for a round Earth have also been shown to fake their evidence. Therefore, the Earth is flat." Argument from adverse consequences An argument that a fact cannot be accepted to be true due to the bad effects it would have if it were true. "If the world was round, then half of the people on Earth would be walking around upside down and they'd get dizzy. So, the world is flat." Argument from authority Argument that we should believe a given "expert" based merely on the authoritative position that expert holds or due to his extensive experience or formal qualifications. "Professor Magnus has a degree in Earth Science and has written 3 books on the subject, and he says that the world is flat. Therefore, it is flat." Possibilities include that the "expert" is not an expert in the particular field of discussion, that other experts disagree or that the expert was misinterpreted, taken out of context, or was not being serious in expressing the view. Ad populum / Appeal to popularity Argument that we should accept a proposition because lots of other people accept it. "Polls show that 95% of people believe the world is flat. Therefore, it is flat." Bandwagon fallacy Where a threat of social rejection is substituted for evidence. "If you hold that the Earth is round, then no flat-Earth society will ever let you join. Therefore, the Earth is flat." Appeal to ignorance Arguing that a particular belief is true because you're not aware of any evidence to the contrary. Or that people should accept your conclusion because there's no conclusive evidence either way. "We don't know for sure that the Earth isn't flat. You can't prove that it isn't. Therefore, it is flat." A related fallacy is to assume that if something cannot be explained now then it will be forever unexplainable. Appeal to ridicule An argument in which ridicule or mockery is substituted for evidence. "The Earth is round! That's the most stupid thing I've ever heard. Therefore, the Earth is flat." Appeal to tradition Falsely assuming that something is better or correct simply because it is older, more traditional or has always been done that way. "There's a long and proud history of flat-Earth theories dating back 5000 years, and many famous people have held throughout history that the Earth is flat. Therefore, the Earth is flat." Argument from personal incredulity "I can't understand how the world could possibly be round. Therefore, it is flat." Ad hoc reasoning Introducing new elements into an argument solely to explain away inconvenient points. "Against your argument, people don't fall off the edge of a flat Earth because gravity is different near the edges of the Earth than near the centre. This fact is not commonly recognised." Straw man To set up a straw man is to argue against a position that you create specifically to be easy to argue against, rather than the position actually held by those who oppose your point of view. "These ideas of your about doughnut-shaped Earth or cubical-Earth are ridiculous and have obvious problems. Therefore, the Earth is flat." (The actual argument that was put was that the Earth is round.) Guilt by association Argument that because certain disreputable people believe in A, anybody who believes in A can't be trusted and therefore any arguments made by believers in A must be false. "Hitler believed the world was round, and he was an evil man. Therefore, the world is flat." [size=+1]Fallacies of presumption[/size] Fallacies of presumption base an argument on one or more false (or at least unwarranted or unproven) assumptions. These assumptions are often implied rather than being explicitly stated. False dichotomy Assuming that only two conclusions are possible when in fact there are more than two. "If the Earth isn't flat, it must be shaped like a banana. Clearly, it's not shaped like a banana, so it must be flat." Begging the question Assuming as a premise what you are trying to prove. Or, simply ignoring an important assumption that should really be included as a separate stated premise. "That the Earth is flat is an inherently sensible idea. Therefore, the Earth is flat." Correlation does not imply causation An argument that because A and B are often observed together, A must cause B. " Note that this fallacy can be mis-applied in an attempt to deny all statistical evidence for an actual causal relationship, such as in properly controlled medical trials or where many independent correlations point to a common cause. Post hoc ergo propter hoc Literally "after this, therefore because of this". The argument that just because A happens before B, A must cause B. Non sequitur An argument where the conclusion doesn't follow from the premise. In other words, a logical connection is implied where there isn't one. "Sheep most like to eat grass on flat surfaces. Therefore, the world is flat." Slippery slope Argument that if a moderate conclusion is accepted then a more extreme version of the same conclusion must also be accepted. Usually accompanied by warnings of dire consequences if the unfavoured conclusion were to be adopted. "If it was possible for the Earth to be round then we'd have to accept the possibility that planets could be cubical or doughnut-shaped or pyramidal. Therefore, the Earth is flat." Special pleading Argument of the form: As are generally B. X is an A. But X is an exception to the general rule because of (irrelevant characteristic). "Planets are usually round. The Earth is a planet. But Earth has life on it, and planets with life on them are flat. So the Earth is flat." [size=+1]Statistical fallacies[/size] Hasty generalisation / Inadequate sample size Making an assumption about a whole group or range of cases based on an insufficient number of actual observations. "I looked at 10 different objects. They were all flat. Therefore, the Earth must be flat." Biased sample Presenting some of the available evidence that appears to support your argument while ignoring other evidence that does not. Gambler's fallacy An assumption that departures from the average or from long-term behaviour will necessarily be corrected in the short term. "I've flipped a coin 8 times and got 8 heads in a row. On the next flip, I'll be more likely to get a tail, because I'm due for one." Fallacy of accident Any argument of the form: A's are normally B. X is an A. Therefore X must be B. This ignores the fact that X may be an abnormal example of an A. "Ground and water (especially) are usually relatively flat. The whole surface of the Earth consists of ground and water. Therefore, the Earth is flat."