There are a great many different fallacies and a great many different cognitive biases: probably enough to keep me going for years if I was to discuss one each week on this blog series. What I want to do here, though, is just to consider the question of what fallacies and cognitive biases actually are, and how they relate to each other. This is a contentious enough subject in itself.
A fallacy is normally described as a flaw in reasoning, or a type of mistake whereby people draw incorrect conclusions from the reasons they start with. This would be correct when applied to formal fallacies, but I think incorrect when applied to the more interesting and practically relevant informal fallacies. Here’s a simple example of a formal fallacy (one known as affirming the consequent):
Good Catholics attend mass regularly.
Bridget attends mass regularly.
Therefore Bridget is a good Catholic.
You might think this was quite a reasonable conclusion to draw from the reasons given. However, it is not a necessary conclusion. You don’t have to be a good Catholic to attend mass regularly, and it’s quite possible that Bridget is an uncertain enquirer, or a bad Catholic trying to keep up appearances, or a Religious Studies scholar doing field research into Catholicism. A conclusion that is not necessarily true is not valid, and thus a formal fallacy.
However, it may be reassuring to reflect that the vast majority of arguments we actually use in practice are formal fallacies. Many of them are inductive (see Critical Thinking 2), and even those that are not inductive may be deduction (like the example above) of a kind that is formally invalid but actually reasonably enough most of the time. Formal fallacies are thus of little interest from the practical point of view. Informal fallacies, on the other hand, tell us much more about unhelpful thinking, even though they may actually be formally valid in some instances.
Informal fallacies are just unjustified assumptions: for example, the assumption that some objectionable personal attribute in the arguer refutes their argument (ad hominem); or that there are only two choices in a situation where there is actually a spectrum of options (false dichotomy); or the assumption that using your conclusion as a reason provides an informative argument (begging the question). What is objectionable about arguments involving these moves often depends on the circumstances, and it requires thoughtful judgement rather than just applying black-and-white rules. But that’s also the indication that these fallacies actually matter in everyday life.
Informal fallacies are unjustified assumptions identified by philosophers. The only genuine difference between informal fallacies and cognitive biases, as far as I can see, is that cognitive biases are unjustified assumptions identified by psychologists and often tested through experiment. Psychologists may explain our tendency to make these particular kinds of unhelpful assumptions in terms of the physical, social and evolutionary conditions we emerge from, but in the end these kinds of explanations are less central than the identification of the bias itself. Usually cognitive biases can be ‘translated’ into fallacies and vice-versa. For example, the in-group bias (tendency to favour the judgements of your own group) is equivalent to the irrelevant appeal to the authority of the group (or its leaders), irrelevant appeal to popularity within the group, or irrelevant appeal to tradition in the group, all of which are recognised informal fallacies. The outcome bias, whereby we judge a past decision by its outcome rather than its quality at the time, involves an irrelevant appeal to consequences.
Philosophers and psychologists thus both have very useful things to tell us about what sorts of mistakes we are likely to make in our thinking, and insofar as their different contributions are practically useful, they tend to converge. I would also argue that this convergence of useful theory relates closely to the avoidance of metaphysics (see cognitive biases page). Despite the widespread idea that fallacies are faults in reasoning, they really have nothing to do with reasoning in the strict sense of logical validity. They are all about the unhelpful assumptions we often tend to make.
See if you can identify and describe the type of unhelpful assumptions being made in these video clips. You don’t necessarily need to know the formal titles of the fallacies involved to identify why they are a problem.