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Flaws in Logic: 101

When evaluating people’s arguments or reasoning, or making your own, it is important to be able to spot any flaws in logical thinking. Flaws in logic do not mean that the conclusion is wrong, but merely that the argument itself does not support the conclusion. For instance:

“Oranges are a citrus fruit, therefore the sky is blue”

This logic is obviously flawed, but it does not mean that the conclusion (that the sky is blue) is wrong, but rather the statement of oranges being a citrus fruit is not a valid argument.

Being able to critically analyse your own and other people’s arguments comes in very handy in every day life. With good critical thinking skills, you are able to evaluate the validity of claims presented to you, whether it be a health claim, new technology, policy or otherwise. It will also allow you to explain to others why some claims may not be valid ones, and help you to make effective and unbiased arguments yourself.

Note: This is intended as a basic introduction to logical reasoning and argument formation, it is by no means a comprehensive and exhaustive list of all aspects of logical reasoning.


Is defined as “a proposition supporting or helping to support a conclusion”; it is an assumption you make as a starting point of your argument and then apply logic in order to come to a conclusion. For example:

Premise 1: A = B

Premise 2: B = C

Logical conclusion: A = C


When evaluating arguments, it is important to establish whether or not the premises are true or not. If a false premise is present, then the argument itself is a false one. For example:

Premise 1: pink fluffy elephants that fly exist

Premise 2: pink fluffy elephants that fly are good for transport

Logical conclusion: we can use pink fluffy elephants that fly as a method of transport

The premise that pink elephants that fly exist is a false one, and therefore the logical conclusion is incorrect.

Premises can also be subject to assumptions, especially if there is not a big enough body of evidence to support the premise. For instance, there may be differing evidence whether or not a drug therapy is effective at treating a disease state – therefore, depending on whether or not you assume the drug is effective or not, your logical conclusions may or may not be valid. In this case it is best to state “let’s assume, for argument’s sake, that…” Generally you will have to wait until more information is available clarifying which assumption is the correct one.

Premises can also be un-stated – they are an un-stated major premise. For instance some recommendations used in explicit guidelines for prescribing are based on findings from the Delphi method, a consensus technique which gathers opinions of experts in the field. The authors of such guidelines state that using experts of varying fields (pharmacy, medical practice, specialists, clinical pharmacology, etc…) gives their guidelines validity. The un-stated major premise here is that the opinions of these experts are up-to-date, unbiased, and (somehow) correct.

Occam’s Razor

"Pluralitas non est ponenda sine neccesitate" or "plurality should not be posited without necessity." This is a way of thinking which suggests that the hypothesis or theory which makes the least number of assumptions is most likely true. For instance:


When a person hears the sound of hoofs in Australia, they can assume that the sound is being made by:

  1. Zebras

  2. Horses

  3. Centaur

In this case, Occam’s Razor would suggest that the answer which results in the least number of assumptions is conclusion 2. Horses, as zebras are not frequently found in Australia, and the existence of centaurs has yet to be proven.

Cherry Picking

This is a type of bias where the person making the argument only chooses to examine evidence which supports their hypothesis, and ignores other forms of evidence which contradict it. Good science dictates that the evidence should lead us to our conclusion, not the other way around. A classic example is anti-vaccination groups using “evidence” to support their claims that vaccination cause all sorts of problems from autism, allergies, asthma, to mind control, but completely ignore all the evidence against their claims.

Remember, one trial cannot prove or disprove a hypothesis or theory – it is the body of evidence (i.e.: when many experiments have been done with similar results) which truly confirm or invalidate them.



Logical Fallacies

Simply put, logical fallacies are errors in reasoning. People can use these arguments to support their claim, but because their logic is flawed, they have most likely come to the wrong conclusion.

This list is not exhaustive, but includes some of the more common fallacies seen in health literature, and is a good starting point.

Ad hominem

This type of argument tends to attack the person rather than addressing the argument itself.

“Person X is wrong because they’re rude”

This argument tries to claim that BECAUSE person X is rude, they are wrong – in fact, the rudeness of a person has nothing to do with whether or not their argument is correct. (Note that “Person X is rude” is just a statement, not a fallacy).

Another example is where some tries to discredit an argument by implying that the opposing person has certain unfavourable traits.


“Don’t listen to him, he looks like Hitler”


Ad ingorantiam

This is an argument from ignorance suggesting that something is true because we don’t know if it is true or not.

“We have not yet discovered every animal on Earth; therefore pink fluffy elephants that fly exist”


In fact, in order to make any positive claims, positive evidence is required. Absence of evidence is just that – it means we do not know the answer, nor does it give us the right to make things up.


Appeal to Nature

This type of argument assumes that just because something is “natural” that it is somehow safe, good, or better than something that is “unnatural”.


“Herbs are natural, so they must be safe”


There are two main issues with this type of argument: one - defining what natural is can be difficult; and two - just because something is natural, does not make it inherently good, safe or better than something unnatural.


For example, some may state that “chemicals” are unnatural, such as ammonia and cyanide. These chemicals are found in nature in water, soil, air, fruits and so are “natural”; however, I doubt that anyone would claim that cyanide consumption is good for you!

Argument from Authority

This is an argument based on an “expert” opinion, which assumes that the expert is somehow correct. There will usually be an emphasis on the experts supposed credentials (e.g.: PhDs, medical back-ground, etc…) as well as years of experience. It is true that an expert will have more knowledge than a lay person, and their credentials and years of experience may add some credibility to their statement, but it does not make it fact.

“Dr X, a neurosurgeon with 35 years of medical experience, says that putting pineapples in your ears will help decrease the risk of Alzheimer’s Disease”


Just because a neurosurgeon with many years of experience has made this statement, does not make it true. However, if he bases his statement on scientific studies, then his statement may in fact be valid.


Argument from Antiquity

This is similar to an argument from authority and is commonly used in health literature. It states that just because a belief has been around for a long time, it somehow makes it true.

“Treatment X has been used for thousands of years”

This statement does not prove that treatment X is effective or not, it merely states that it has been used for a long time – nothing more, nothing less.


Argument from Personal Incredulity

This is an argument where the person claims that something is impossible or cannot be true just because the person cannot explain or understand it. The same logic can be used for claiming that something is true just because the person cannot fathom it not being true. For instance:

“I cannot imagine how X can be true, therefore it is false”


“I cannot imagine X not being true, therefore it must be true”


Argument from Popularity

This is similar to the argument from authority but it assumes that if many people believe in an argument it makes it true, and is another popular logical fallacy in health literature. For instance:


“Many people use treatment X”


This statement does not prove that treatment X is effective or not, it merely states that it has been used by many people – nothing more, nothing less.


Begging the Question

This is where a proposition which requires proof, is assumed without the necessary proof stated. A classic example of this in “CAM” proponents is:


“Why are health professionals hiding the truth about our medications from us?”


This question is asking why are health professionals hiding the truth about medications, but assumes that doctors are hiding anything to begin with. In fact, it is common for health professionals like pharmacists to disclose as much as possible about a medication to the patient in order to educate them.


Confusing Association with Causation

This is assuming that just because two things occur together, that one causes the other. This is why in science we cannot show causation until an experiment is specifically designed to do so; otherwise we can only demonstrate a correlation or association between two variables.

“Increased shoe size increases reading and writing skills of children”


These two variables (shoe size and reading/writing skills) generally occur together because as children grow older, their shoe size increases, and their brain develops which improves reading and writing skills. Therefore, although increased shoe size and improved reading/writing skills occur together, increased shoe size does not cause improved reading and writing skills.


Note: correlations are a good starting point for many scientific studies. Causation is usually shown through an intervention study, or with well thought out epidemiological studies.

For more hilarious examples go here.


Confusing currently unexplainable with unexplainable

Just because a phenomena is not currently explainable with current scientific knowledge and technologies, does not mean it will never be explainable or that it should be explained with unscientific explanations (i.e.: ones that are outside the realms of science, or explanations for which there is no evidence).

There are many drugs for which we did not have a formulated mechanism of action, but with increasing knowledge about the human body, we have been able to explain how they work. An example of this was the action of the active molecule found in cannabis, THC, and the discovery of the endocannabinoid system.

For drugs which we do not have a proposed mechanism of action it is appropriate to state just that – we do not know the mechanism of action. It would be inappropriate to state that:

“We do not know how this medication works; therefore magic fairies throw pixy dust on us every time we take the drug to make it work”


False Analogy

Usually analogies are a very useful way to make complex concepts easy to understand – e.g.: the lock and key analogy for enzymes.

A false analogy assumes that two things are similar when in fact, the two may not be similar at all in the image painted. For instance:

“Your body is like a pipe system – it needs to be cleaned out regularly to get rid of all the build up junk”


This argument is commonly used for colon irrigation therapy where the colon is irrigated in an attempt to get rid of build up “toxins”. Although there are some similarities between a “pipe system” and the human body (e.g.: blood vessels) this analogy is a false one as the “junk” in the colon is intentional – it is the substance which the body thinks is not necessary for absorption.


False Continuum

This is where it is assumed that there is no difference between two extremes which lie on a continuum or that the differences between the two are meaningless. There are fuzzy definitions for some diagnoses such as conditions within the Autism spectrum, but that does not mean that they are all the same thing.


False Dichotomy

This assumes that there are only two possible answers to a given questions, which is usually a false assumption. It also assumes that if one of the possible answers is false, that the other one must therefore be true. For instance:

“If you are not with us, you are against us”


This assumes that being neutral, a third option, does not exist, and thus creates a false dichotomy.



This is commonly seen in the health literature, where a person will apply one set of criteria or rules for one argument/claim/belief/position but not others. For instance, we tend to demand evidence for efficacy and safety for most prescription drugs, but do not apply the same rules to many CAMs.



This means “does not follow” – this is an argument where the conclusion does not follow a true premise. A logical relationship is implied where there is none.

“Oranges are an orange colour; therefore, pink fluffy elephants exist”


Post-hoc ergo propter hoc

This argument assumes that because there is a temporal relationship between two random events, that the first event caused the second event.


“I sneezed and then there was an earthquake; therefore, my sneeze caused the earthquake”

This is similar to the (false) logic Wakefield used in his study to link MMR vaccines and autism – he suggests that because the MMR vaccines were given first and then autistic symptoms appeared, they must have caused the autistic symptoms (never mind that he completely fabricated the results!).


Special pleading

This is when a person introduces new elements to an argument in order to fix them. This type of reasoning usually occurs in order to dismiss negative results.

“You cannot see the pink fluffy flying elephants because you are not a true believer”


This introduces a new element: that only true believers in pink fluffy flying elephants are able to perceive them.


“Western science cannot comprehend or properly evaluate Eastern medicine”


This assumes that Eastern medicine (where medicine is a science by definition) is somehow fundamentally different at a basic scientific level to Western science, and therefore Western science cannot prove or disprove the efficacy of Eastern medicine. Unfortunately, science is science where ever you are in the world.



Straw Man argument

The “straw man” is an argument against which it is easy to argue. The person creates this argument and argues against the straw man rather than the argument their opponent really holds.

“You do not believe that we should sell medication without evidence in pharmacies. Would you put an old lady with dementia, dieing of cancer in distress by taking her homeopathic medication away??”


The real issue here is that medications without evidence are being sold to start off with. However, this person uses a straw man argument (the old lady) to make it easy to argue in favour of selling medications with no evidence, such a homeopathy, in pharmacies.



This type of argument uses circular reasoning – that is, the premise is the conclusion without any further information. Simply put it takes the structure of A = B, therefore B = A

This type of argument may be difficult to spot at first, but when the premise, argument and final conclusions are broken down, it usually becomes apparent. For instance:

“This medication is first line because it is the best available option”

By definition, a first line medication is the medication which is considered the best available option; therefore this statement is a tautology.


The Moving Goalpost

This is where the person arbitrarily keeps changing the criteria they require for “proof” of an argument out of the evidence that currently exists. The analogy is the image of a football going into a goal during the footy – new evidence may meet the criteria required as proof of a concept (GOAL!) but the person changes the criteria, thus “moving the goal post” and the new evidence no longer meets the criteria. This is a word tactic commonly used by anti-vaccination proponents; for example they claimed that thimerosal, a mercury containing preservative, caused autism. However, despite the removal of thimerosal from all vaccines, they still claim that vaccines cause autism but now blame it on various “toxins”.


Tu quoque

This is the “you too” fallacy. It attempts to justify that two wrongs make a right – in other words, just because someone else does something wrong, their action is therefore justified. A recent article discussing the ethics of using “CAMs” with no evidence to them used this fallacy – in the “yes its ethical” response, the author states

“One apparent hurdle is that much CAM currently lacks high-quality evidence. However, this should not be taken as proof that a given CAM is ineffective or harmful. To place this in context, it is estimated that as little as a quarter of conventional medicine is based on level-1 evidence”

Just because some “conventional” medications do not have high levels of evidence for use (note that this does not mean that there is no evidence) does not mean that CAMs should be used for which there is no evidence what-so-ever.

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