Top 20 Logical Fallacies

A while back I began a series Faulty Thinking about logical fallacies. I didn’t get round to finishing it. But the other day I found this list of twenty at the The Skeptics Guide to the Universe website. Here they are:

Ad hominem

An ad hominem argument is any that attempts to counter anothers claims or conclusions by attacking the person, rather than addressing the argument itself. True believers will often commit this fallacy by countering the arguments of skeptics by stating that skeptics are closed minded. Skeptics, on the other hand, may fall into the trap of dismissing the claims of UFO believers, for example, by stating that people who believe in UFO’s are crazy or stupid. 

Ad ignorantiam

The argument from ignorance basically states that a specific belief is true because we don’t know that it isn’t true. Defenders of extrasensory perception, for example, will often overemphasize how much we do not know about the human brain. UFO proponents will often argue that an object sighted in the sky is unknown, and therefore it is an alien spacecraft. 

Argument from authority

Stating that a claim is true because a person or group of perceived authority says it is true. Often this argument is implied by emphasizing the many years of experience, or the formal degrees held by the individual making a specific claim. It is reasonable to give more credence to the claims of those with the proper background, education, and credentials, or to be suspicious of the claims of someone making authoritative statements in an area for which they cannot demonstrate expertise. But the truth of a claim should ultimately rest on logic and evidence, not the authority of the person promoting it. 

Argument from final Consequences

Such arguments (also called teleological) are based on a reversal of cause and effect, because they argue that something is caused by the ultimate effect that it has, or purpose that is serves. For example: God must exist, because otherwise life would have no meaning. 

Argument from Personal Incredulity

I cannot explain or understand this, therefore it cannot be true. Creationists are fond of arguing that they cannot imagine the complexity of life resulting from blind evolution, but that does not mean life did not evolve. 

Confusing association with causation

This is similar to the post-hoc fallacy in that it assumes cause and effect for two variables simply because they are correlated, although the relationship here is not strictly that of one variable following the other in time. This fallacy is often used to give a statistical correlation a causal interpretation. For example, during the 1990’s both religious attendance and illegal drug use have been on the rise. It would be a fallacy to conclude that therefore, religious attendance causes illegal drug use. It is also possible that drug use leads to an increase in religious attendance, or that both drug use and religious attendance are increased by a third variable, such as an increase in societal unrest. It is also possible that both variables are independent of one another, and it is mere coincidence that they are both increasing at the same time. A corollary to this is the invocation of this logical fallacy to argue that an association does not represent causation, rather it is more accurate to say that correlation does not necessarily mean causation, but it can. Also, multiple independent correlations can point reliably to a causation, and is a reasonable line of argument. 

Confusing currently unexplained with unexplainable

Because we do not currently have an adequate explanation for a phenomenon does not mean that it is forever unexplainable, or that it therefore defies the laws of nature or requires a paranormal explanation. An example of this is the “God of the Gapsâ” strategy of creationists that whatever we cannot currently explain is unexplainable and was therefore an act of god. 

False Continuum

The idea that because there is no definitive demarcation line between two extremes, that the distinction between the extremes is not real or meaningful: There is a fuzzy line between cults and religion, therefore they are really the same thing. 

False Dichotomy

Arbitrarily reducing a set of many possibilities to only two. For example, evolution is not possible, therefore we must have been created (assumes these are the only two possibilities). This fallacy can also be used to oversimplify a continuum of variation to two black and white choices. For example, science and pseudoscience are not two discrete entities, but rather the methods and claims of all those who attempt to explain reality fall along a continuum from one extreme to the other. 

Inconsistency

Applying criteria or rules to one belief, claim, argument, or position but not to others.  For example, some consumer advocates argue that we need stronger regulation of prescription drugs to ensure their safety and effectiveness, but at the same time argue that medicinal herbs should be sold with no regulation for either safety or effectiveness. 

Non-Sequitur

In Latin this term translates to “doesn’t follow”. This refers to an argument in which the conclusion does not necessarily follow from the premises. In other words, a logical connection is implied where none exists. 

Post-hoc ergo propter hoc

This fallacy follows the basic format of: A preceded B, therefore A caused B, and therefore assumes cause and effect for two events just because they are temporally related (the latin translates to “after this, therefore because of this”). 

Reductio ad absurdum

In formal logic, the reductio ad absurdum is a legitimate argument. It follows the form that if the premises are assumed to be true it necessarily leads to an absurd (false) conclusion and therefore one or more premises must be false. The term is now often used to refer to the abuse of this style of argument, by stretching the logic in order to force an absurd conclusion. For example a UFO enthusiast once argued that if I am skeptical about the existence of alien visitors, I must also be skeptical of the existence of the Great Wall of China, since I have not personally seen either. This is a false reductio ad absurdum because he is ignoring evidence other than personal eyewitness evidence, and also logical inference. In short, being skeptical of UFO’s does not require rejecting the existence of the Great Wall. 

Slippery Slope

This logical fallacy is the argument that a position is not consistent or tenable because accepting the position means that the extreme of the position must also be accepted. But moderate positions do not necessarily lead down the slippery slope to the extreme. 

Special pleading, or ad-hoc reasoning

This is a subtle fallacy which is often difficult to recognize. In essence, it is the arbitrary introduction of new elements into an argument in order to fix them so that they appear valid. A good example of this is the ad-hoc dismissal of negative test results. For example, one might point out that ESP has never been demonstrated under adequate test conditions, therefore ESP is not a genuine phenomenon. Defenders of ESP have attempted to counter this argument by introducing the arbitrary premise that ESP does not work in the presence of skeptics. This fallacy is often taken to ridiculous extremes, and more and more bizarre ad hoc elements are added to explain experimental failures or logical inconsistencies. 

Straw Man

Arguing against a position which you create specifically to be easy to argue against, rather than the position actually held by those who oppose your point of view. 

Tautology

tautology is an argument that utilizes circular reasoning, which means that the conclusion is also its own premise. The structure of such arguments is A=B therefore A=B, although the premise and conclusion might be formulated differently so it is not immediately apparent as such. For example, saying that therapeutic touch works because it manipulates the life force is a tautology because the definition of therapeutic touch is the alleged manipulation (without touching) of the life force. 

The Moving Goalpost

A method of denial arbitrarily moving the criteria for “proof” or acceptance out of range of whatever evidence currently exists. 

Tu quoque

Literally, you too. This is an attempt to justify wrong action because someone else also does it. “My evidence may be invalid, but so is yours.” 

Unstated Major Premise

This fallacy occurs when one makes an argument which assumes a premise which is not explicitly stated. For example, arguing that we should label food products with their cholesterol content because Americans have high cholesterol assumes that: 1) cholesterol in food causes high serum cholesterol; 2) labeling will reduce consumption of cholesterol; and 3) that having a high serum cholesterol is unhealthy. This fallacy is also sometimes called begging the question.

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3 Responses to Top 20 Logical Fallacies

  1. Carlos says:

    hmmm, I must disagree about the reductio ad absurdum. I mean, I agree with you in what you say, but that’s exactly why that way of porving things is not bad itself.
    A properly used reductio ad absurdum won’t lead to any mistake, it is not a fallacy, in fact it is one of the very few real ways of proving things.
    Maybe you should put only 19 fallacies

  2. Carlos says:

    yesterday you asked me for a ad adsurdum reasoning which were good. And i didn’t know what to answer because all i could thought was maths. But in fact it’s very easy.

    Let’s do an easy one.

    We want to prove that a person is a woman. And we only know that a person is either man or woman, and that men always have penis.

    Then we do reductio ad absurdum. We suppose our person is a man, but as we cannot find his penis we “reach a contradiction” which is being man and not having penis. Therefore what we supposed was wrong, and therefore again by reduction ad absurdum it must be a woman.

    this is perfect, is not only true, but it is true because of that reasoning.

    Is that maybe clarifying?

  3. molivam42 says:

    I think I need to investigate this more.

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