Part 1. Section 4a

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4. Logical Fallacies


So far we have concentrated on types of arguments which can either guarantee the truth of the conclusions given all true premises (valid arguments) or can provide good reasons to accept the truth of their conclusions (strong arguments based on induction). We now turn to a set of loosely connected types of bad reasoning which have occurred throughout the ages in public debates and arguments which are collectively known as logical fallacies.


Logical Fallacy

Definition: A logical fallacy is an error in reasoning which has been particularly persuasive or historically common place.


In other words, logical fallacies are not good arguments from a logical standpoint but turn out to still be persuasive for one reason or another. Here we do not consider fallacies which occur by virtue of the form of the argument, which are known as formal fallacies in other treatments, but rather only discuss fallacies based on certain characteristics which have 'psychological' appeal. To illustrate this, let's consider some actual examples of one particular fallacy type.


90% of the people in several independent polls report accepting the claim that O.J. Simpson was indeed guilty of the murder of his wife and Ron Goldman, so he must be guilty.


No one believes that Kara is telling the truth, hence she is lying.


In 1720 almost no one thought that slavery based on race was wrong, hence it wasn't in 1720.


In early Spring of 1912, everyone believed that the ocean liner RMS Titanic was impossible to sink, hence anyone travelling on the Titanic will arrive safely at their destination.


Notice that all of these arguments make a conclusion based only on what people believe or believed, but long experience tells us that belief alone is not sufficient to establish the truth or probability of a claim - one needs evidence or reasons for one's belief. Arguments that make inferences based on the beliefs of many people alone are a type of fallacy known as ad populum arguments (or 'appeal to the populace' arguments). The fact that popular belief alone is not sufficient to establish the truth of a claim is well known and readily agreed on by almost anyone pressed with the question and given time to consider several examples. Why then do ad populum fallacies exist? The answer to this question takes us beyond the study of logic, but we venture to make two observations which may help explain the continued popularity of so many fallacies.

  1. People mentally 'add' premises to fallacious arguments which make them stronger.
  2. There is an emotional appeal to such arguments which is strong enough to override critical thinking.

To give some motivation for 1, when hearing the first argument above it may be the case that there is an unspoken assumption by those who hear this argument that the people polled know about the evidence given in the 'trial of the century' and those polled were basing their conclusions on that evidence. Notice if this fact is indeed true, then the argument is modified to something of the form, "Most people polled knew the arguments in the OJ Simpson case and in several independent polls report … ". However, the argument given does not make this claim, and in evaluating it, one should be careful not to add information not given in the original argument.

For reason 2 we just note that there is often an observed conflict between one's emotions and logic. As a matter of fact, the ancient Greek Sophists were accused of exploiting this very difference in teaching the art of rhetoric rather than logic.

Whatever the reason for the continued use of fallacies, they are still very much with us and one's reasoning skills are greatly improved by having some familiarity with the most common fallacies. For the purposes of argument classification, all of the fallacies which follow, with the exception of circular reasoning, should be classified as weak arguments. Circular reasoning is an example of a valid argument which is valid by virtue of the fact that one of its premises is equivalent to the conclusion.

Below we present some common fallacies arranged in no particular order with a test yourself quiz at the end of each group. In the Section Review, there is also a Flashcard activity where you can review the fallacy names and definitions.