Tag Archives: logical fallacies

Faulty Premises 2

In my last post I discussed some of the more common and simplistic logical fallacies. I would recommend going back through that post if you are unsure what a logical fallacy is, as this post will assume that the reader has some prior knowledge.

 

Argument from Authority

This can be a tricky fallacy because we all know that often arguments such as those put forth in research papers rely on the referencing of relevant authorities in various fields to support them. Quoting Stephen Hawking when talking about the physics of black holes, for example would not be a fallacy because he is considered an expert in that field and there is consensus among experts on much of what he says. Thus his words can be trusted to a reasonable degree.

The fallacy occurs when someone uses the authority of an individual that is not a reliable expert on a given subject. For example it would be inappropriate to use the views of an engineer to support an argument about biology. No matter how much of an expert they might be about engineering, this has no bearing on the veracity of their views on biology.

Another form of this fallacy occurs when someone appeals to the views of a relevant expert, and holds them to be true whilst ignoring the fact that there is no consensus among other experts in that field. For example one might quote a particular scientist and use them to support a particular view on the evolution of language, taking their word as truth, whilst completely ignoring the fact that many other experts disagree.

 

Quote Mining

This fallacy is similar to the argument from authority in that it uses quotations often taken from well-known figures. The difference here is that the quotation is taken out of context, or important information is left out. Take a look at this quote from Charles Darwin:

“But, as by this theory, innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?”

This appears to the reader as though Darwin is saying that there is a problem with evolutionary theory—the lack of transitional fossils. Quotes like this are frequently used by creationists in an attempt to show that even Darwin had doubts about his own theory, however if one puts this quote into context, one can see that a crucial piece of information is left out:

But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth? It will be more convenient to discuss this question in the chapter on the Imperfection of the Geological Record; and I will here only state that I believe the answer mainly lies in the record being incomparably less perfect than is generally supposed. The crust of the earth is a vast museum; but the natural collections have been imperfectly made, and only at long intervals of time.

When put into context we can see that Darwin did not view this question as an insurmountable problem with no conceivable answer, its just that the following sentences were removed to give this impression. The use of quotations in such a misleading manner can be difficult to spot (especially if one does not have access to the original material to look it up). As a general rule of thumb, one should be weary of people using quotes from experts that appear to contradict their known views or appear to be saying something extraordinary.

 

Misleading Use of Polls and Stats

Many people use stats and polls to support their arguments, however these can often give misleading results. Where possible, one should always examine these to see whether the results may have been biased in some way. Lets use an imaginary statistic to illustrate this: 7 out of 10 British people state that that British weather is miserable.

Lets say you look into this poll and find that it was only conducted among a small number of people in Northern Scotland. The first issue would be sample size; a poll that uses a small number of individuals cannot be extrapolated to be representative of an entire nation.

The second area of potential bias is in the location of the survey, Northern Scotland is generally colder than southern parts of Britain and thus it might be expected that there would be more people who are unhappy about the weather in that region. This cannot be considered representative of the entire nation.

You might also find out that the survey was conducted in mid-January when the winter is at its worst and most miserable, a factor likely to affect people’s answers on the matter. You then discover that the survey was conducted among farmers, who as a general rule spend more time outside than people in other professions—another way in which the answers might be biased.

Finally you hear that the actual question asked was “do you agree that the weather in Britian is miserable?”—when asked in this way the question is presupposing that the weather in Britian is miserable and might thus influence the answers given.

These are a few examples in which polls and surveys can be biased. It is advisable to try to ask the following questions, when presented with such results:

  • Was the sample size large enough to be representative?
  • What factors in the method of the survey might lead it to be biased? (location, individuals asked, method of asking etc.)
  • Was the question leading or presumptuous in any way?

Of course many polls and statistics can be enlightening and are carried out in ways that eliminate bias, but it is always useful to view these results in a critical manner.

 

That’s all for now. Hopefully I have given some insight into various kinds of logical fallacy. I would be interested to hear some feedback, comments and criticisms about these posts, and ask a specific question; would you like me to continue posting about logical fallacies, or should I move on to discussing other topics?

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

www.logicalfallacies.info/

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