Types of fallacies

In these blogs, now and then I write about fallacies. I think that this is important since the way we think has a big impact on the way we behave; on our private behaviour and our public behaviour. Therefore, it is better to avoid mistakes. As for public behaviour, one can think of political decisions and juridical verdicts, for instance. Especially, mistakes in juridical decisions can have dramatic consequences; for example, that an innocent suspect is sentenced to long prison terms, if not to the death penalty. Political decisions can lead to war or peace, so the reasoning that leads to such decisions must be sound. As for private behaviour, reasoning errors can have an impact on private life, such as wrong or too expensive purchases or voting for a president who doesn’t represent your interests, although you thought so. This time, I don’t want to discuss a special fallacy, but I want to give some background information.
Basically, there are two types of fallacies: Formal fallacies and informal fallacies. Both types of fallacies are based on incorrect deductive reasoning, but the difference is that in a formal fallacy, the conclusion doesn’t follow from the premises, because the structure or form of the reasoning is not correct, while in an informal fallacy the conclusion doesn’t follow from the premises because of the content of the argumentation. An important example of a formal fallacy we often come across in daily life is the “post hoc ergo propter hoc” fallacy (“post hoc ergo propter hoc” literally means “after this, so because of this”). An example: “If it has rained, the street is wet. This morning, when I woke up, the street was wet, so it has rained tonight.” This need not be true, for maybe a leaking tank lorry has passed. See this and this blog for more examples.
Although formal fallacies often happen in daily life, I think that informal fallacies are more frequent, especially in political discussions and other public discussions. Basically, there are three types of informal fallacies:
- Fallacy of presumption. It occurs, so Arp et.al (p. 22), “when an argument rests on some hidden assumption – it could be an unknown factor, a condition, set of circumstances, state of affairs, or idea – that, if not hidden, would make it clear that the assumption is not sufficient to be able to reason to (draw, infer) the conclusion.” The conjunction fallacy, discussed in one of my blogs (see link), is a case in point.
- Fallacy of relevance. It occurs, so Arp et.al (p. 25), when the premise or premises are irrelevant to the conclusion, even though they may appear so, because of an appeal to psychological or emotional relevance. Best known is the ad hominem fallacy (“playing the man instead of the ball”), discussed by me, for example, here and here. Also the “red herring” and “straw man” are fallacies of relevance.
- Fallacy of ambiguity. A fallacy of this type, so Arp et al. (p. 26), relies on some ambiguity (vagueness, obscurity, non-clarity) in wording or phrasing, the meanings of which shift/change to various degrees of subtlety during the course of the argument. The Sorites paradox is a case in point (“Take away again and again a grain of sand from a pile of sand; when is it no longer a pile?”) (see, for example, this blog). How often doesn’t it happen that the meaning of a word shifts during a discussion, or that it is so vague that you can use it for “any” conclusion by way of speaking?
This is how we often reason or how we try to convince others. But when we are doing so, we are on the wrong track.

Source
This blog is mainly based on the Introduction to Robert Arp; Steven Barbone; Michael Bruce (eds.), Bad arguments. 100 of the most important fallacies in Western philosophy. Oxford, etc.: Wiley Blackwell, 2019.