Logical Fallacies

Discussion in 'General Philosophy' started by James R, May 17, 2011.

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  1. James R Just this guy, you know? Staff Member

    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."
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  3. glaucon tending tangentially Registered Senior Member

    Thanks for the work James.
    I've been pondering over whether or not to formulate such a quick reference guide myself for some time now.

    I've made this a sticky.
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  5. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

    Can they be numbered, so that we can just reference the numbers in our posts?

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  7. glaucon tending tangentially Registered Senior Member

    Uh, no.

    Nor is this going to be a thread for discussing logical fallacies....

    Reference material only please.
  8. Randwolf Ignorance killed the cat Valued Senior Member

    While this may not be a discussion thread, I would like to pose a question as to inductive vs deductive.

    James has provided the more comprehensive formal definition, but is there not still a component referencing "from the specific to the general" and "from general to specific"?

    Or is that only part of the lay definition?

    My hope here is to clarify the reference material, after which this post may / should be deleted.
  9. glaucon tending tangentially Registered Senior Member

    Your suspicion is correct Randwolf; "from the specific to the general" and "from general to specific" is the old laic means of explaining induction and deduction. This harkens back to ancient Aristotelian syllogistic logic, whereas the more logically defined expressions were derived after greater formalization of Logic, starting with Peirce and Frege.

    This is a decent place to start:

    Philosophypages - logic
  10. Cifo Day destroys the night, Registered Senior Member

    Not to go overboard, but Wikipedia has a "List of Fallacies" entry that contains four times the number of entries here.

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    And then there's other stuff like Wikipedia's "List of Cognitive Biases" that's also fairly lengthy. :bugeye:

    Last but not least, I belong to another forum that prohibits calling someone on a logical fallacy because it is supposedly far too stifling. (Although this still stumps me.

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  11. glaucon tending tangentially Registered Senior Member


    Suffice it to say, Wikipedia is hardly a qualified Primary (or Secondary...) Source...

    That's just silly.
    Logical Form exists for a reason.
  12. James R Just this guy, you know? Staff Member

    As I said:

    "I present here a selected list of some common logical fallacies, with examples."

    I've also linked to a huge taxonomy of fallacies in the OP.
  13. wynn ˙ Valued Senior Member

    If you have enough of the contentious spirit of the Schopenhauerian art of being right, you can call anything a logical fallacy, and demonstrate it as such - and anything anyone were to counter you, you could again classify as a logical fallacy.

    Philosophical discussion is mainly an act of goodwill and charitability. But they are so easy to throw out, and this is when the race to point out logical fallacies begins. And it is a race toward nonsense.
  14. Asguard Kiss my dark side Valued Senior Member

    James I have some issues with your list, in some situations these may not be invalid arguments. For instance the slippery slope may not be the best ethics argument but it is concided a valid argument. Fir instance if we stop locking up gays for being gay the next thing they will want us to let them marry. Ok so what's wrong with that but its still a valid argument (also happenes to be true and I'm glad it is)

    Then there is apeals to authority, "you need to be doing 30:2 with that CPR because that's the current guidelines from the Australian ressucitation council" or there are no randomized control trials on the use of adrenilin but its judged best practice by the experts. This is concidered the lowest grade of evidence in Med but its not invalid unless superseeded by higher grade evidence

    Hell vertually the whole DSM and ICP are that grade of evidence, doesn't make them invalid
  15. Gustav Banned Banned

    i remember the glory days when my fallacies were stickied in phil
    damn you all!

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  16. Cifo Day destroys the night, Registered Senior Member

    I greatly appreciate James' list and the freedom to use it, and I wish it was even shorter, minimal.
  17. glaucon tending tangentially Registered Senior Member

    People, as I have pointed out, this is not a thread for discussion.

    If you feel pressed to discuss a fallacy, or fallacies in general, start such a thread.

    Again: this thread is for posting references only.
    Stay on topic.

    Fair warning.
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