Logical Fallacies 13 Common Errors in Logic P in the book

Post Hoc, Ergo Propter Hoc 1 Latin for “after this, therefore because of this.” This fallacy occurs when one event is assumed to have caused a second event just because the two events occurred in sequence. Example: On Tuesday afternoon I got my haircut. On Tuesday evening my goldfish died. My goldfish died because I got my haircut. Obviously, these events are unrelated. Another example is luck or coincidence. Any cause-effect relationship is difficult to establish.

Only-Cause Fallacy 2 It is an oversimplification the name a single cause for a complex situation. Pause here and read the example of an only-cause fallacy on p According to the argument only one thing causes the absence of peace in the world: lack of fear of destruction by nuclear force. Look for the only-cause fallacy whenever the causes – and solutions – of complicated problems are discussed. A complex problem will have neither a simple cause or single solution.

Non Sequitur 3a Latin for “It does not follow.” Whenever a conclusion does not logically and necessarily follow from the premises or evidence, a non sequitur occurs. Inductive Non Sequitur  Lisa has chicken pox.  Wendy has chicken pox.  Charlie has chicken pox.  Therefore, the schools should be closed because there is an epidemic of chicken pox.  (3 cases are not an epidemic, so conclusion is not logical.

Non Sequitur 3b Deductive Non Sequitur  All living things require water  An automobile requires water  Therefore, an automobile is a living thing.  obviously an automobile is not a living thing; therefore, the 2 nd requirement for a valid conclusion (establishing an individual as a subset of the major set) is violated. Even if the conclusion and minor premise were reversed, the conclusion would be false because the minor premise would be false.

Hasty Generalization 4 A conclusion based on too small a sampling is called a hasty generalization. Example – if you decided you hated all Mexican food because you hated the first taco you ever tasted. Example – judging a person based on first impressions. Remember generalizations should only be made after a large and random sampling.

Stereotype 5 A stereotype is a hasty generalization about groups of people. Stereotypes are almost always negative. Racial, religious, economic, ethnic, and gender stereotypes are the most common. Combat stereotypes by recognizing them in daily life, advertising, jokes, and everyday speech and avoid them or speak out against them. Also combat stereotypes by getting to know and respect people from every kind of racial, religious, economic, and ethnic background.

Unreliable Authority (Ipse Dixit) 6 Ipse Dixit (meaning “He said it”) is the fallacy of quoting a person who is not an expert in the field being discussed. Examples – an athlete discussing the nutrition of cereals or a TV doctor giving medical advice. The example about the comedian on p. 249 talking about Sunshine Village is a fallacy because he does not live there and cannot be an expert on whether or not it is a fun place.

Irrelevance or Distraction 7 This fallacy consists of including facts, reasons, or opinions that are not really part of the argument. They may touch on it, but they do not relate to it directly. Pause here and read the argument on p Items b and f are not relevant to the argument on p. 249 because the members of the lawyer’s family are not experts in the field, and the biographical data about a female judge has nothing to do with the number of female lawyers and female judges.

Attacking the Person and Not the Issue (Ad Hominem) 8 Ad Hominem “against the man” and often appear in election campaigns and debates when one candidate attacks the other’s personal life, rather than his or her stand on the issues. This type of strong personal attack is also known as “poisoning the well.” In the argument on p. 250 Candidate X is guilty of Ad Hominem because he only discusses his opponent’s personal finances, not his opponent’s stand on inflation.

Argument by Analogy 9 An analogy is a comparison in which two things are shown to have at least on quality in common. Although an analogy comparing two things may be logical and work, the analogy alone proves nothing. The argument on p. 250 about school for three-year-olds is weak because it is supported only by an analogy, which is a fallacy.

False Analogy 10 A comparison that is shown, on close examination, to be farfetched is called a false analogy. The analogy comparing the lives of bees to that of humans on p. 251 is a false analogy because there are too many differences between the two that are not taken into account. In addition, if argument by analogy is a fallacy, argument by false analogy is certainly a fallacy.

Begging the Question or Circular Thinking 11 Both of these names identify the same fallacy: arguing that a conclusion is true without providing any evidence or reasons. If you examine the argument on p. 251, you will see that the writer is arguing that students be allowed to leave school before the ninth grade – but he doesn’t provide any reasons. He simply restates in other words that he thinks the law is unjust and that students should be allowed to leave school before 9 th grade. It is a little like having the opening and closing sentences of a paragraph, but leaving out the middle of the paragraph where the writer proves the main idea.

A Priori 12 A Priori means “based on a previous statement.” It is similar to circular thinking, in which the writer assumes that the statement is true and expects the reader or listener to believe that it is true simply because the writer says so. Both the arguments on p. 251 are examples of a priori reasoning. They would be fine as summary statements, but no reasons or evidence are given as support.

Excluded Middle or Either-Or 13 In this fallacy, the speaker gives only 2 alternatives, usually polar opposites, and ignores all other alternatives in the middle which are less extreme. For example, “Marry me tomorrow, or never see me again” is an example of and excluded middle/either-or argument. The speaker only offers two extreme options and ignores all the other less extreme options in between. The argument on p. 252 does the same.