The word fallacy is sometimes used as a synonym for any kind of position that is false or deceptive, and sometimes it is applied in a more narrow sense to a faulty process of reasoning or to tricky or specious persuasions. We will use "fallacy" in the latter sense so that one may say a fallacy occurs where a discussion claims to conform to the rules of sound argument but, in fact, fails to do so.

We have chosen to examine chiefly the notorious fallacies. In our descriptions we try to make it clear what the particular case before us is, and, in the examples, we try to illustrate one particular fallacy at a time, without entirely sterilizing the sample. But fallacies reinforce each other, and in a weak argument there is apt to be more than one thing wrong. In almost all cases the examples are derived from the world we live in. They are representative of actual discussion, so they are often disguised for obvious reasons, though perhaps not out of all recognition, and trimmed down to size.

Logic is the defense against trickery. The kinds of argument with which logic deals are the reasonable ones. Mistakes are possible, even frequent, in applying the forms of logical argument, and these mistakes are regarded as fallacies, many of them having been noted as early as Aristotle. We shall wish to guard against them. But the most common fallacies today are of a very different sort. It is a small comfort to know that an argument is entirely logical, that it validly derives its conclusion from its premises, and that all the rules of the syllogism, or whatever, are observed to a nicety, if it turns out that the premises are frauds, snares, delusions. There are brilliant tricks for getting people to accept all sorts of false premises as true (some of these tricks have been spotted since the time of ancient Greece), and these tricks of argument are so prevalent that even when people realize that something is being pulled on them, they tend to let it pass.

Few can hope to become immune to all the tricks of persuasion since, like viruses, there are too many of them. People are daily exposed to appeals to blind faith, self interest, fear, prejudice, fancy. This book cannot discuss persuasion in all its variety and complexity, but it can attempt to describe and illustrate some of the most dangerous strains.