Tuesday, May 21, 2013

Logical fallacies classification - formal and informal fallacies


Logical fallacies are classified in many different ways since there is still no agreement between thinkers for that matter. What are logical fallacies? The best definition of a fallacy in logic is an argument with poor reasoning (critical thinking rules can easily unmask flawed conclusions). The most basic classification divides logical fallacies into two main groups – formal fallacies and informal fallacies.

Formal fallacy simply describes an error in a logical form of an argument. One of the most common formal fallacies is known as proof by example. It occurs when the speaker (or writer) uses an example to make overgeneralisations (this fallacy is often used in propaganda). Consider the following example:

Jews I know are rich therefore all Jews earn above average.

This statement is a clear oversimplification – even if it happens to be true the argument still is an example of formal fallacy. You should always remember that fallacies do not address the conclusion itself but judge the whole argument regardless of what the conclusion is. Note that some arguments which are not fallacious contain a false conclusion! It is illustrated by the example:

No man can live more than 100 years. Therefore Jiroemon Kimura is less than 100 years old. 

Conclusion is logical but since the premises of the argument are false they lead (logically) to the false conclusion (Mr. Kimura is in fact 116 years old).

Informal fallacy definition is that while it is logically valid, its technical structure (such as use of words) is misleading or unclear which makes the argument fallacious. The common example of informal fallacy is  equivocation which occurs when the same word used in the argument carries two separate meanings (appeal to novelty is also an informal fallacy). Consider the following argument:

You can use sharp things to cut paper. Christopher has a sharp mind – therefore you can cut paper with Christopher’s mind.

The absurdity of this argument is more than obvious but the argument is valid as the conclusion follows from the premises. What is more, both premises are true. However, the argument is fallacious due to equivocation – an example of informal fallacy. Notice that the same reasoning applied in this argument may does not make it fallacious:

You may get fat if you eat many sweets. Christopher eats many sweets therefore he may get fat.

This argument is very reasonable and not fallacious despite the fact it uses the same reasoning pattern as the previous argument. Therefore you cannot distinguish between fallacious and non fallacious arguments by considering a reasoning pattern only.

This classification of fallacies is of course very basic. In fact, within the two main groups – formal and informal fallacies there are many other subgroups. I will introduce them in the next posts so you can have a clear view on the whole “tree” of fallacies. If you enjoyed the post don’t hesitate to like Mind and Philosophy Facebook page!



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