Faulty Premises 1

Faulty Premises Part 1

Last time we discussed how to dissect an argument, by finding its premises and the conclusion. I also spoke briefly about non-sequiturs—something which is known as a logical fallacy. There are many different kinds of logical fallacy, all of which when used weaken the strength of an argument. In this post I will outline some of the more common logical fallacies, not only so that you can spot them in the arguments made by others, but perhaps more importantly, you can spot them in your own.

A logical fallacy is, as the name implies, a use of false logic to support a conclusion. The best way to highlight exactly what a logical fallacy is, is through showing examples of them.


Ad hominem

An ad hominem is when something derogatory is said of a person making an argument or holding a position in place of an actual argument. An example of this is as follows:

Creationists are stupid therefore creationism is false.

The statement ‘creationists are stupid’ by itself is not a logical fallacy (though not a particularly nice thing to say), it is only when the derogatory statement is used as a premise for the conclusion that it is a logical fallacy. Even if the statement is valid, it does not follow from it that the conclusion is true, therefore it is a fallacy.


Argument from ignorance

(Argumentum ad ignorantiam)

This fallacy is when someone argues the following:

We don’t know that X is false

Therefore X is true

A few common example of this would be:

Science doesn’t know everything, therefore it is reasonable to conclude that the supernatural exists.

Telepathy is possible – we don’t know everything about the human brain after all.

All that can really be derived from an absence of knowledge is the conclusion; we don’t know. A specific area of ignorance does not make a positive claim plausible or even possible. When one makes a positive claim such as ‘the supernatural exists’ or ‘telepathy is possible’, it needs to be supported with positive evidence. An area of ignorance does not allow one to fit anything that one likes into that gap.


Argument from personal incredulity

This fallacy is committed when someone states that they cannot believe how something could be true and then uses this to support their conclusion that this is something is false. A common example of this would be the creationist argument:

I do not understand how something as complex as the human eye could have evolved by chance, therefore evolution is not true.

The logical error committed here is that one’s personal inability to comprehend something is grounds for dismissing it. In reality this is not true, and whilst you personally might not be able to understand something, this does not mean that nobody can. In the above example, there have been numerous instances in which scientists have explained precisely how the eye could evolve via gradual incremental steps (I’d recommend Richard Dawkins’ book Climbing Mount Improbable for such an example). ‘I don’t understand X’ means nothing when used as a premise.


Post hoc ergo propter hoc

(Correlation does not imply causation)

Post hoc ergo propter hoc is Latin for ‘after this, therefore because of this’. This fallacy is made when someone assumes a causal link between two things simply because one occurred before the other. In formal terms this fallacy is written thus:

X occurred before Y

Therefore X caused Y

An example of this fallacy would be:

Since the MMR vaccine was introduced diagnosis rates for autism have risen, therefore the MMR vaccine causes autism.

The argument is assuming that there is a link between these two events; the introduction of the MMR vaccine and the rise in diagnoses of autism. However these events following each other does not necessarily mean that one caused the other. Therefore in order to accept the conclusion as valid the person presenting the argument would need to demonstrate that there is indeed evidence of a causal link between the two events that extends beyond the fact that one event preceded the other.

There are cases in which correlation does imply causation, however in these cases there is more evidence to support the claim of causation than the fact that the two phenomena occurred around the same time. X occurring before Y by itself is not sufficient grounds for claiming that X caused Y.


I shall end this here for fear of rambling on too much. I will endeavour to highlight more examples of logical fallacies in the next few posts of this series. Until then I can point the interested reader towards the following links, should you wish to know more:

Wikipedia’s list of logical fallacies

The Nizkor Project – Fallacies


I hope you have enjoyed this post. Feel free to respond with any comments, questions, ideas, suggestions, disagreements or whatever in the discussion thread.

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