Strictly speaking, a fallacy is a mistake in reasoning. One commits a fallacy when the reasons advanced or accepted in support of a claim fail to justify its acceptance. A fallacy can be committed either when one is deciding whether to accept a claim on the basis of a fallacious argument with which one has been presented, or when one is presenting the fallacious argument oneself.

A fallacious argument or inference is one in which there is an inappropriate connection between premises and conclusion. Almost all fallacies fall under one of the following two types:

• Formal fallacies. Sometimes the inappropriate connections are failures of logical connection, for example in the case of the fallacy of affirming the consequent; here the argument or inference is neither deductively valid nor inductively forceful, even where all implicit premises have been made explicit. It is simply a logical mistake.

• Substantive fallacies. Sometimes the inappropriate connections involve reliance on some very general unjustified assumptions or inferences. We need only make these premises explicit in order to see that they are false and unjustified. What distinguishes a fallacious argument of this kind from an ordinary unsound argument is that the implicit, false premise will be of a very general nature, having nothing specifically to do with the subject-matter of the argument. What this means will become apparent when we turn to examples.

The vast majority of fallacies that we encounter in everyday texts and speech are substantive fallacies, but some are formal fallacies, and a few are neither formal nor substantive. Arguments embodying formal or substantive fallacies are necessarily unsound, but not all fallacious arguments are actually unsound. Since fallacies are many and various in this way, it would be tedious and distracting to formulate a precise definition. We will simply take fallacies to be mistakes in reasoning that arise because of inappropriate connections between premises and conclusions; we will identify the most commonly encountered fallacies, and we will demonstrate different strategies for dealing with them.

It is important at the outset to note the general point, however, that a fallacious argument can have true or false premises: Simply having false premises does not make an argument fallacious. Nor does having true premises guarantee that an argument is not fallacious.

Furthermore, a proposition accepted on the basis of a fallacious argument may turn out to be true as a matter of actual fact. Suppose that someone reasons as follows:

There is blood on the candelabra. And if Colonel Mustard killed the victim with the candelabra, then there would be. So Colonel Mustard must be the murderer.

Laid out in standard form the argument is thus:

P1) If Colonel Mustard killed the victim with the candelabra, then there is blood on the candelabra. P2) There is blood on the candelabra.

C) Colonel Mustard killed the victim.

Now suppose that, in fact, the conclusion is true: Colonel Mustard is the murderer. Still, the reasoning that takes us to that conclusion is fallacious (the fallacy is the fallacy of affirming the consequent). It is not legitimate to infer on the basis of PI and P2 that Colonel Mustard is the murderer. Even if both premises were true, it would be possible for the conclusion to be false: for example, someone else could have killed the victim with the candelabra. Knowledge of the truth of those premises, just by themselves, would not be sufficient to infer the conclusion, even if that conclusion were actually true. If you were to be fooled by such an argument, you would end up with a true belief, but for mistaken reasons.

Like linguistic phenomena and rhetorical ploys, the best way to become acquainted with the different types of fallacies considered in this chapter is to practise identifying and analysing them. As they are attempts to persuade by argument, you need to reconstruct them in standard form and then use techniques of argument analysis and assessment to demonstrate the ways in which they are fallacious.

And one last thing to bear in mind before we work through specific types of fallacies: as we have already mentioned, many types of fallacious argument are effective as rhetorical ploys. Someone might be aware that their argument commits a fallacy, but will use it to try to persuade us because they are aware of its rhetorical power: they are aware, that is, that it does tend to persuade people. Common examples include the use of ad hominem, majority belief and slippery slope fallacies to rhetorical effect.

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