Fallacies are mistakes of reasoning, as opposed to making mistakes that are of a factual nature. If I counted thirty people in the room when there were in fact thirty-one, then I have made a factual mistake. On the other hand, if I found out that one of the people in the room could speak English and I immediately jumped to the conclusion that everyone could, then this mistake of reasoning was indeed a fallacy, and in this particular case the fallacy of over-generalization.
Broadly speaking, we can divide fallacies into four kinds:
- Fallacies of inconsistency– Proposing or accepting a position that is inconsistent or self-defeating, such as believing that every rule has an exception. (If the rule is correct, it would have an exception, but that means there is a rule without an exception!)
- Fallacy of inappropriate presuppositions– Cases where we have an assumption or a question presupposing something that is not reasonable to accept in the relevant conversational context. Asking whether human nature is good or evil presupposes that there is such a thing as human nature and that it must be either good or bad. But these assumptions might not be correct and if no adequate justification is offered then the question might not be an appropriate one.
- Fallacies of relevance– Cases where an irrelevant assumption is used to defend a conclusion. For example, suppose someone argues that it is acceptable to eat meat because meat is tasty. But this reason is irrelevant. Whether it is ok to kill and eat a living thing should not depend on whether it is tasty or not. Presumably it is not OK to eat babies even if they turn out to be delicious.
- Fallacies of insufficiency– Cases where the evidence supporting a conclusion is insufficient or weak. There are many such examples, e.g. over-generalization, the naturalistic fallacy,mistaking correlation for causation, etc.
Fallacies are closely related to cognitive biases, which are persistent and widespread psychological tendencies that can affect rational and objective judgments. For example, human beings tend to be over-confident of their abilities, and we tend to think that biases affect other people but not ourselves. Moreover, our objectivity can easily be affected by our emotions or mindset, or even irrelevant factors in the environment. There are lots of interesting and surprising findings in the psychology literature about such biases. It is not easy to avoid biases, but here are a few things we can do:
- Read more about cognitive biases so we can be more alert of them.
- Be honest and clear about the reasons for our decisions. Record our major decisions in a journal. Review our track record periodically to see if we keep repeating the same mistakes.
- Use relevant data and evidence to guide our decisions. Talk to experts or people who know more than we do.
- Actively seek out opinions that are different from yours and consider them carefully.