22 March 2011

Logical Fallacies

One of the most useful skills in the skeptical repertoire is the ability of identify logical fallacies. A logical fallacy is an error in reasoning that is not supported by logic. They generally take the form of a non-sequitur, an argument with a conclusion that does not follow from the premise. It is important for skeptics to recognise fallacious arguments not only so that we can know when people are making invalid arguments based on flawed logic, but also so that we don’t fall into the trap of making such errors ourselves. Following is a list of 27 of the most common logical fallacies you may come across:

Ad Hominem /poisoning the well
An ad hominem argument is one that attempts to invalidate the arguments or conclusions of others by attacking the person instead of addressing the actual argument. A similar fallacy, poisoning the well, is an argument that assumes a person to be wrong because s/he has some unsavoury quality or holds an unpopular belief. Such arguments fail to address the opponent’s argument and consequently cannot be used to draw valid conclusions about the argument. 

Skeptics are all close-minded.
UFO believers are all crazy.
Creationists are just stupid.
She is wrong because she’s a scientologist.

Appeal to nature/naturalistic fallacy
An appeal to nature is the fallacious argument that assuming we can define something as “natural” or “unnatural” (often decided by arbitrary criteria), what is natural is good (or at least better) and right, and what is unnatural is bad (or at least worse) and wrong. There is no factual basis to support such assumptions. These arguments occasionally take the form of “we are/aren’t meant to…”.

Natural alternatives are better for our health than pharmaceutical drugs.
Organic fruit and vegetables are the healthier option.
Homosexuality is unnatural and wrong.
We are meant to eat meat/not meant to drink milk.

Appeal to Popularity
An appeal to popularity is the argument that because many people hold a certain position, that position must be true. There have been many incidences in the past where many people have held beliefs that we now know not to be true such as the belief that the universe revolved around the Earth, or that diseases spontaneously generated. Such examples show that the number of people who hold a certain position has no bearing on how true it is.

Christianity is the most popular religion in the world, so it must have some truth to it.
Millions of people use complementary and alternative medicine, so it must work.

Appeal to Tradition
An appeal to tradition is the argument that simply because a belief is old or traditional, it must be true, or better than newer beliefs. Such an argument is fallacious as the age of a belief has no bearing on its accuracy. For example, that the universe revolves around the Earth was a belief held since ancient times, however we now know that this is not true. Another example is that bloodletting was a popular form of medical treatment for over 1000 years that we now know does not provide any benefit and is actively harmful.

Herbal remedies have been used for thousands of years, so they must be superior to modern medicine.
People have believed in god for thousands of years, so god must exist. Why else would the belief persist for so long?

Argument from Authority
An argument from authority is one that asserts that because a person with considerable authority believes something to be true, it must be true. No matter how many qualifications a person has, it does not follow that that person is always going to be correct. Often the argument is reversed as an ad hominem argument that because a person has few qualifications, their arguments must be false. 

Some NASA scientists believe in UFOs.
James Randi argues that homeopathy doesn’t work, so it must be true.
He doesn’t have a college education so he must be wrong.

Argument from Final Consequences
An argument from final consequences can take several forms. The first assumes a reversal of cause and effect, arguing that events are caused by the ultimate effect or purpose it has, as though the outcome was the “goal” all along. It may also take the form of concluding that a premise is true or not based on whether it results in desirable or undesirable consequences. 

The Earth has all the properties required for life to exist, so it must have been designed specifically to support life.
Animals reproduce so that their genes will be passed down to following generations.
Belief in evolution leads to immoral, animalistic behaviour, so evolution is inherently wrong.
Belief that we were all created by god leads to mutual respect for one another as we are all god’s creatures.

Argument from Ignorance/Ad Ignorantium/God of the Gaps

An argument from ignorance is one that makes conclusions based on what we don’t know. It can take the form of the argument that because we don’t know that something isn’t true, it must be true. Such an argument creates a false dichotomy by excluding the position that there is insufficient evidence to support the theory in question. This fallacy crops up frequently in skeptical circles as many pseudociences are unfalsifiable because you can’t prove a negative.  The god of the gaps argument is a subtype of the argument from ignorance as it claims that if we don’t know what caused something, god must have done it. This argument is logically flawed as if we don’t know what caused something, all we can conclude that it is currently unknown, rather than leaping to the assumption that god did it.

It can’t be proven that God doesn’t exist, therefore he exists.
It can’t be proven that all UFO sightings aren’t aliens, therefore UFOs are probably aliens.
We still don’t know what caused the big bang, so God must have done it.

Argument from Incredulity/imagination
Closely related to arguments from ignorance, an argument from incredulity is an argument based on what one is capable of imagining, and takes the position that because it is too incredible to be true, it can’t be true, or I can’t imagine how it isn’t true/it is obvious that it is true, so it must be true. Fortunately the truth is not limited by what we are/are not capable of imagining what is/isn’t true. 

I can’t imagine how the human eye could have possibly evolved, so evolution can’t be true.
It is obvious that something as complex as the human eye was made by a creator, so creationism must be true.

Circular Reasoning/Begging the Question/Tautology
Also known as begging the question and tautology, circular reasoning is any argument where the conclusion is the same as its premise. Such reasoning is fallacious as it attempts to pass the conclusion off as evidence of itself without introducing any real evidence. Circular reasoning is not always immediately apparent as the premise and conclusion may be worded differently (see second example below). 

We know the Bible is the word of God because it’s written in the Bible.
Acupuncture works because it uses needles to improve the flow if chi in the body.*
*This is circular reasoning because the definition of acupuncture is using needles to allegedly improve the flow of chi in the body.

Confusing Correlation and Causation
This is the argument that because two variables are correlated, one must have caused the other. This is a problem because it assumes direction (instead of A causing B, B might have caused A), and ignores other extraneous or confounding variables that may have influenced both A and B.
Playing violent videogames causes violent behaviour in children.
Being taller causes better handwriting in children.

Confusing Unexplained with Unexplainable
This is the argument that because something is not yet satisfactorily explained by science, it is unexplainable. Because we do not yet have an adequate explanation for a phenomenon, it doesn’t mean that we never will. This argument is often used to justify the god of the gaps argument that asserts that if something is unexplained, then god must be responsible. 


Scientists are baffled how the Great Pyramid was constructed with such precision; it will forever be a mystery.

We will never know what caused the big bang.

False Analogy
An analogy uses inductive reasoning to argue that because two objects/persons/phenomena are similar they must share similar characteristics, so knowledge of one can be used to draw conclusions about the other. A false analogy is where the premise that those two objects/persons/phenomena are similar is incorrect, meaning that knowledge of one cannot logically be used to draw conclusions about the other.

Because a complex object such as a car has a creator, a complex object such as an organism must have a creator as well.*
*This statement is fallacious as the premise implies that a car is similar to an organism when it is not. While both cars and organisms are undoubtedly complex, they are fundamentally different because organisms are systems that are capable of reproduction, development, and growth, whereas a car is an inert piece of metal machinery.

False Continuum 
This is an argument that because there is no clear point that separates two extremes, no meaningful destinction can be drawn between the two. For example, just because there is no clear point where a person can be categorised as short or tall, it does not follow that there is no meaningful difference between being short and being tall.

All religions are just cults.
Everyone is skeptical about some things; therefore everybody/nobody can be labelled a skeptic.

False Dichotomy
Misleadingly reducing a large set of possibilities to only two options creates a false dichotomy. Such an argument is an oversimplification as it assumes that there are only two possibilities, and ignores the positions that lie between both possibilities along a continuum. Such arguments may lead to incorrect assumptions that if one possibility is false, the other must be true. A false dichotomy can also be used to claim that things can be categorically labelled as one thing or another when that might not necessarily be so.

Conventional medicine does not yet have a cure for cancer; alternative medicine offers the only solution.
Evolution cannot be true therefore creationism must be true.
Which is better, conventional, or alternative medicine?*

*This statement is fallacious as it assumes that all treatments can be classified as either conventional or alternative medicine, when in reality there can be much overlap between the two. For example, yoga began as a conventional Eastern form of treatment that gradually has been accepted by conventional practitioners, such that it is neither categorically conventional or alternative.

Goodwin’s Law
Goodwin’s law is a specific type of attempt to poison the well by drawing similarities between the position of a person or theory, and the positions of Adolf Hitler and the Nazi regime. This is such an extreme, anomalous example that it cannot offer any meaningful criticism. 

Hitler was an atheist (even though he wasn’t), so atheists must have poor moral standards.
The Nazis’ experiments of eugenics are based on the principles of evolution, so evolution is inherently wrong.

Inconsistency Fallacy
It is fallacious to apply one set of rules and standards to one belief, theory, or claim, but not to others. Such inconsistencies reveal biases in the standards of evidence deemed acceptable to support different positions. 

Using carbon dating as evidence of the age of an archaeological discovery alleged to be Noah’s ark, but claiming that carbon dating is an unreliable tool to estimate the age of fossils.
Arguing that there should be stronger regulations for conventional medicines to ensure their safety and efficacy, but that herbal remedies should be sold without regulations for safety and efficacy.

Lottery Fallacy
The lottery fallacy is a misuse of statisics that generally argues that a particular phenomenon has such a low probability of happening that because it did happen, it cannot be by chance alone. This argument is often used by creationists in an attempt to argue that because the Earth’s conditions are perfect for life, the Earth must have been designed for us. Asking what is the probability that this particular phenomenon should occur? is asking the wrong question. The law of truly large numbers states that with a large enough sample size, any unlikely thing is likely to happen. A more meaningful question is what is the probability that any unlikely phenomenon should occur, given a large enough sample size? 

How likely is it that I will win the lottery? vs. how likely is it that anyone will win the lottery?
How likely is it that the Earth just happened to have the right conditions for life vs. how likely is it that any planet in the universe has the right conditions for life?

Moving the Goalposts/Raising the Bar
Moving the goalposts is when one arbitrarily changes the criteria required to prove a claim out of reach of the available evidence. Such an argument is fallacious as it fails to accept evidence against one’s own position by forever demanding more evidence. 

There is not enough fossil evidence of transitional forms.* 
*There are many examples of transitional fossils. What constitutes “enough”?

No True Scotsman Fallacy
The no true Scotsman fallacy is the use of fallacious reasoning to retain an assertion in the face of counterexamples. This is achieved by arbitrarily manipulating the counterexample so that it no longer conflicts with the original asssertion, rather than correctly rejecting the original assertion. 

Person 1: All Scotsmen like haggis.
Person 2: My father is a Scotsman and he doesn’t like haggis.
Person 1: Well then he isn’t a true Scotsman.
Psychics who make errors when predicting the future just aren’t true psychics.

Post Hoc Ergo Propter Hoc
A Post hoc argument is one that proposes a causal link between two events because one occurred before the other. It argues that because A occurred before B, A must have caused B. Two events can co-occur without one causing the other, so without more evidence than a co-occurrence, an assumption that one caused the other is fallacious. 


My child was diagnosed with autism following her vaccinations, so the vaccines must have caused the injury.
My Power Balance wristband I put on this morning caused me to score extra goals today.
The 2004 Indian Ocean earthquake and tsunami must have been caused by the supermoon that occurred 2 weeks before.

Reductio ad Absurdum
Reductio ad absurdum is the argument that if the premise leads to one or more absurd conclusions, the premise must be false. While this can be a logically legitimate argument, it is fallacious to manipulate logic in order to force an absurd conclusion. 

If I don’t believe in UFOs, then I cannot believe in the Great Wall of China either as I personally haven’t witnessed either of them.*
*This is fallacious as it excludes all evidence other than personal eyewitness accounts. It is not necessary to reject the existence of the Great Wall of China to be skeptical of the existence of UFOs.

Regression Fallacy
Related to post hoc reasoning, this is failing to take into account the natural fluctuations of things when attributing causes to them. Petrol/gas prices, our health, and subjective feelings of pain inherently fluctuate between mid, high, and low points. Ignoring these fluctuations may lead to self-deception by making causal inferences between unrelated events and these natural fluctuations. Complementary and alternative therapists often take advantage of this tendency to ignore natural fluctuations in health and subjective wellness by stating that their patients may feel worse before they feel an improvement, covering the whole range of natural fluctuations. 

My lucky rabbit’s foot helped to improve my stock prices.
A spinal manipulation by my chiropractor worked because my sinuses cleared up within 5 days.
Single Cause Fallacy
The single cause fallacy is the oversimplified assumption that an event has a single cause when in fact there may be a number of independent and/or interrelated causes that contributed to the event.

Heavy metal music causes teenagers to become aggressive and withdrawn.
Junk food advertising marketed towards children has resulted in a large increase in childhood obesity.

Slippery Slope Fallacy
Related to the false continuum fallacy, a slippery slope argument states that a small first step may lead towards a chain of events culminating in some significant effect. Such an argument is fallacious if it is accepted without justification that A will lead to Z. This fallacy can also take the form of the assumption that if a person accepts a position, the extreme of that position must also be accepted. 

The introduction of a secular alternative to scripture classes is the first step in the path to eradicating religion entirely from schools.
Legalising marijuana will lead to more heroin addicts.
If you are a Christian, you must also accept creationism.

Special Pleading/Ad-Hoc Reasoning
Special pleading is generally the arbitrary introduction of new elements or standards to an argument or position such that it remains valid in the face of counter evidence. This is a fallacious as it disregards counter evidence by inventing unsupported, unjustified explanations. 

The true nature of homeopathy cannot be measured by a randomised, controlled, double-blind study.
Dowsing rods don’t work in the presence of skeptics due to their negative energy.

Straw Man
A straw man argument is responding to or attacking an exaggerated version of an opponent’s claims. This creates the illusion of having addressed the opponent’s position by substituting it with a superficially similar claim that is easier to attack. Such an argument is fallacious as it misrepresents the opponent’s position while failing to address their real position. 

We shouldn’t spend more money on helping the poor because handouts only create more poverty.*
*The straw man argument here is changing the original argument from “more funding” to “more handouts”.
Evolutionists argue that we are all animals so we may as well just act like animals and have no moral standards.

The Exception that Proves the Pule
Arguments that make generalisations based on anomalous examples are fallacious as they ignore the rest of the data that suggests such rare examples are outliers that cannot justifiably be generalised to the rest of the data. 

Evolution is a fraudulent field full of hoaxes such as Haekel’s embryos, the Piltdown man, and the Nebraska man.

The Don Lindsay Archive
A special thanks to Steven Novella of The Skeptics’ Guide to the Universe, whose list of logical fallacies was the main inspiration for this post

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