The basics of logic

Being able to critically analyse your own and other people’s arguments comes in very handy in everyday 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.

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.

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: Dragons that fly exist
Premise 2: Dragons that fly are good for transport

Logical conclusion: We can use dragons that fly as a method of transport

The premise that dragons 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 unstated—in which case they are an ‘unstated major premise’. For instance, some recommendations used in explicit guidelines for prescribing medication 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 unstated major premise here is that the opinions of these experts are up-to-date, unbiased, and (somehow) correct.

Occam’s Razor

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. Centaurs

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.

Occam’s Razor is also sometimes called parsimony.

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 causes all sorts of problems from autism, allergies, asthma, to mind control, but completely ignoring 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 and is a good starting point.

Ad hoc

‘For the purpose’; the practice of adding more premises (unsupported ones) to save a belief or position.

Ad hominem

‘Against the man’ or ‘sticks & stones’ i.e. “You wouldn’t believe him because he’s a…”  One well-known example is to compare person or group to Hitler or Nazis!

Ad ignorantiam

The ‘for all you know’ fallacy. We’re not absolutely sure it’s true/false so it must be false/true. Fails when reasonable steps have been taken to establish the truth/falsity of the claim. Thereafter a tentative conclusion may be drawn.

UFO proponents are probably the most frequent violators of this fallacy. Almost all UFO eyewitness evidence is ultimately an argument from ignorance—lights or objects sighted in the sky are unknown, and therefore they are alien spacecraft.

Intelligent design is almost entirely based upon this fallacy. The core argument for intelligent design is that there are biological structures that have not been fully explained by evolution—therefore a powerful ‘intelligent designer’ must have created them.

In order to make a positive claim, however, positive evidence for the specific claim must be presented. The absence of another explanation only means that we do not know—it doesn’t mean we get to make up a specific explanation.

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”

Appeal to authority

The ‘says who?’ fallacy. Appeal to authority must be only ever secondary to strong premises. The ‘authority’ figure must be someone recognised and respected by the majority of their peers.

Ad populum

‘Appeal to popularity’ or the ‘bandwagon fallacy’. Can a billion people be wrong? YES! Popular opinion is not a justification—the majority has been wrong before e.g.” the earth is flat”, “slavery is OK”, etc.

Appeal to emotions

The ‘appeal to pity’ fallacy.

Appeal to force

The ‘scare-tactic’ fallacy.

Begging the question

Circular reasoning, in which the conclusion is presented as a premise. E.g. the Bible is true because it says so in the Bible.

Confirmation bias

When the only tool you have is a hammer, everything looks like a nail. We tend to focus on ‘hits’ and ignore ‘misses’, like people who see psychics and only remember the things they got right.

Common cause

When A and B happen simultaneously, assume that one causes the other when in fact C caused either or both of them.

Confusing cause & effect

The ‘cart before the horse’ fallacy.

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”

Disanalogy/ False analogy

A false analogy is an argument based upon an assumed similarity between two things, people, or situations when in fact the two things being compared are not similar in the manner invoked. Saying that the probability of a complex organism evolving by chance is the same as a tornado ripping through a junkyard and creating a 747 by chance is a false analogy. Evolution, in fact, does not work by chance but is the non-random accumulation of favourable changes.

False dichotomy

This fallacy (aka the false dilemma and the ‘black or white’ fallacy) occurs when one is presented with only some of all the alternatives to choose from. . 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.

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.


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, dragons exist”

Post hoc

The post hoc ergo propter hoc (‘after this therefore because of this’) fallacy is based upon the mistaken notion that simply because one thing happens after another, the first event was a cause of the second event. Post hoc reasoning is the basis for many superstitions and erroneous beliefs.

Reductio ad absurdum

In formal logic, this 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 UFOs does not require rejecting the existence of the Great Wall.

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 dragons because you are not a true believer”

This introduces a new element: that only true believers in dragons 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.


To distort or misrepresent an argument one is trying to refute is called the straw man fallacy. It doesn’t matter whether the misrepresentation or distortion is accidental and due to misunderstanding the argument or is intentional and aimed at making it easier to refute. Either way, one commits the straw man fallacy.

In other words, the attacker of a straw man argument is refuting a position of his own creation, not the position of someone else. The refutation may appear to be a good one to someone unfamiliar with the original argument.


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

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 it’s 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.

*CAM=Complementary & Alternative Medicine