Über die Autorin bzw. den Autor

Keith Parsons (Houston, TX), is professor of philosophy at the University of Houston, Clear Lake; the author of God and the Burden of Proof, among other books; and the editor of The Science Wars: Debating Scientific Knowledge and Technology.

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RATIONAL EPISODES

Prometheus Books

Contents

ACKNOWLEDGMENTS........................................................................................7PREFACE FOR INSTRUCTORS................................................................................9PREFACE FOR STUDENTS (You Really Should Read It).......................................................13CHAPTER ONE: What Is Logic?............................................................................17CHAPTER TWO: Sentential Logic Basics...................................................................37CHAPTER THREE: Sentential Logic Proofs.................................................................57CHAPTER FOUR: More Sentential Logic: Contradictions, Tautologies, and Assumptions......................77CHAPTER FIVE: Predicate Logic Basics...................................................................89CHAPTER SIX: Proofs in Predicate Logic.................................................................101CHAPTER SEVEN: Probability: The Basic Rules of Life....................................................123CHAPTER EIGHT: The Theorem of Dr. Bayes................................................................137CHAPTER NINE: Probability Illusions: Why We Are So Bad at Inductive Reasoning..........................155CHAPTER TEN: "Studies Have Shown" ... Or Have They?....................................................169CHAPTER ELEVEN: Inference to the Best Explanation......................................................189CHAPTER TWELVE: Rhetoric: The Art of Persuasion........................................................203CHAPTER THIRTEEN: Won't Get Fooled Again (Fallacies and Other Foibles).................................219CHAPTER FOURTEEN: Spin, Spin, Spin (Or How Not to Be a Sucker).........................................237CHAPTER FIFTEEN: Everything You Know Is Wrong! Pseudoscience and Bogus Scholarship.....................253GLOSSARY...............................................................................................273INDEX..................................................................................................289

Chapter One

Why do we believe the things we do? We hold many of our beliefs because we were taught them at an early age and never thought to question them. We believe many things, even important things, for the slightest of reasons. A TV commercial we heard twenty years ago, which has long since faded from conscious memory, may still prompt us to prefer brand "X" over brand "Y." We believe many things because they flatter our vanity, soothe our fears, or pander to our biases. Politicians know this and that is why they are so good at pushing our buttons. Occasionally, though, we live up to our reputations as rational creatures and try to find out whether something is really so. That is, we try to think clearly and objectively about things and strive to base our beliefs on the best evidence available and not infer more than the evidence authorizes us to believe. In other words, we try to be like the wise man, who, as David Hume says, "proportions his belief to the evidence."

Also, we frequently try to influence other people's beliefs. Since humans are social creatures, and we have to act collectively to get important things done, and since actions depend on beliefs, we have to be interested in other people's beliefs. Another reason to be interested in other people's beliefs is that beliefs have consequences, often serious ones. If someone believes that people like you are evil, and that anyone who dies in the act of killing people like you will receive rich rewards in paradise, then you had better be on guard. Because beliefs matter, many people exert considerable effort to influence others' opinions.

Politicians, pundits, and advertisers often try to influence people's beliefs by manipulating them in various ways—by playing on their fears or prejudices, for instance. Sometimes, though, we try to influence other people's beliefs by reasoning with them rather than merely manipulating them. When we attempt to persuade people rationally we offer them arguments. When we draw conclusions from arguments or evidence we make an inference. Logic is about argument and inference. We study logic to learn how to distinguish good arguments from bad ones and to learn how to construct good arguments. Concomitantly, logic teaches us which inferences we should or should not draw from given arguments or evidence.

When we offer an argument to someone, we are trying to get that person to accept some conclusion. The reasons we offer in support of those conclusions are called premises by logicians. Since we communicate with language, and since language is organized into sentences, our arguments will be groups of sentences. In particular, our arguments will consist only of declarative sentences, that is, sentences that assert that something is either so or not so. Other types of sentences, those that ask questions or express commands, cannot constitute arguments (though they might sometimes suggest or imply arguments, e.g., when somebody asks "Do you really plan to wear that?" he or she signals willingness to argue that you shouldn't wear that). So an argument consists of a set of declarative sentences, one of which is the conclusion, and with one or more premises. The following is an argument; the premises are marked by the letter "P" and a number indicating which premise it is, and the conclusion is marked by the letter "C":

P-1: Few good people enter politics, and those who do soon cease to be good.

P-2: Joan Smith has decided to enter the mayor's race.

C: Either Joan Smith is not a good person, or she will soon cease to be one.

You may agree with the conclusion of this argument, or you may find it overly cynical. The point is that anyone offering you such an argument intends that the premises give you reasons for accepting the conclusion. You also often hear people saying that premises support a conclusion, or give grounds for the conclusion, or justify the conclusion. These are just different ways of saying that the premises are supposed to give us good reason to think that the conclusion is true.

Sometimes the premises of an argument really do support the conclusion, and sometimes they do not. A good argument is one where the premises support the conclusion; a bad argument is one where the premises fail to support the conclusion. Another way of putting it is that a good argument is one where the conclusion follows from the premises, but a bad argument is one where the conclusion does not follow from the premises. Logic gives us norms for distinguishing good arguments from bad.

Here is a good argument: P-1: All firemen are employees of the Department of Public Safety.

P-2: Jarrod is a fireman.

C: Jarrod is an employee of the Department of Public Safety.

Here is a bad argument:

P-1: All tenured faculty members of the History Department are PhDs in history.

P-2: Erin is a PhD in history.

C: Erin is a tenured faculty member in the History Department.

What makes the first argument good and the second one bad? In the first argument, the premises support the conclusion in the strongest possible way, namely, if the premises of the first...