[F01] What is a fallacy

Fallacies are mistakes of reasoning, as opposed to making mistakes that are of a factual nature. If I counted twenty people in the room when there were in fact twenty-one, then I made a factual mistake. On the other hand, if I believe that there are round squares, I am believing something that is inconsistent. This is a mistake of reasoning, and a fallacy, since I should not have believed something inconsistent if my reasoning is good.

In some discussion, a fallacy is taken to be an undesirable kind of argument or inference. For example, a certain textbook explains "fallacy" as "an unreliable inference". In our view, this definition of fallacy is rather narrow, since we might want to count certain mistakes of reasoning as fallacious even though they are not presented as arguments. For example, making a contradictory claim seems to be a case of fallacy, but a single claim is not an argument. Similarly, putting forward a question with an inappropriate presupposition might also be regarded as a fallacy, but a question is also not an argument. In both of these situations though, the person is making a mistake of reasoning since he is doing something that goes against one or more principles of correct reasoning. This is why we would like to define fallacies more broadly as violations of the principles of critical thinking, whether or not the mistakes take the form of an argument.

The study of fallacies is an application of the principles of critical thinking. Being familiar with typical fallacies can help us avoid them. We would also be in a position to explain other people's mistakes. There are different ways of classifying fallacies. Broadly speaking, we might divide fallacies into four kinds.

  • Fallacies of inconsistency: cases where something inconsistent or self-defeating has been proposed or accepted.
  • Fallacies of inappropriate presumption: cases where we have an assumption or a question presupposing something that is not reasonable to accept in the relevant conversational context.
  • Fallacies of relevance: cases where irrelevant reasons are being invoked or relevant reasons being ignored.
  • Fallacies of insufficiency: cases where the evidence supporting a conclusion is insufficient or weak.

We shall discuss these fallacies in the next few tutorials.

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