chapter 8 formal fallacies and fallacies of language

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  • Slide 1
  • Chapter 8 Formal fallacies and fallacies of language
  • Slide 2
  • Three Formal Fallacies Affirming the Consequent Denying the Antecedent Undistributed Middle 2015 McGraw-Hill Higher Education. All rights reserved. 2
  • Slide 3
  • AFFIRMING THE CONSEQUENT Whenever an argument is in this form: If P, then Q. Q. Therefore, P. It is an invalid argument. 2015 McGraw-Hill Higher Education. All rights reserved. 3
  • Slide 4
  • AFFIRMING THE CONSEQUENT If Jane is a member of a sorority, then she is female. Jane is female. Therefore, Jane is a member of a sorority. An Invalid Argument! 2015 McGraw-Hill Higher Education. All rights reserved. 4
  • Slide 5
  • DENYING THE ANTECEDENT Whenever an argument is in this form: If P, then Q. Not-P Therefore, Not-Q It is an invalid argument. 2015 McGraw-Hill Higher Education. All rights reserved. 5
  • Slide 6
  • DENYING THE ANTECEDENT If Howard passed the final, then he passed the course. Howard did not pass the final. Therefore, Howard did not pass the course. An Invalid Argument! 2015 McGraw-Hill Higher Education. All rights reserved. 6
  • Slide 7
  • THE UNDISTRIBUTED MIDDLE When someone assumes that two things related to a third thing are related to each other, as in: All cats are mammals. All dogs are mammals. Therefore, all cats are dogs. 2015 McGraw-Hill Higher Education. All rights reserved. 7
  • Slide 8
  • THE UNDISTRIBUTED MIDDLE Takes several forms: X has features a, b, c, etc. Y has features a, b, c, etc. Therefore X is Y. Another form is: All Xs are Ys. This thing is Y Therefore, this thing is X. [We saw this form in the previous slide.] 2015 McGraw-Hill Higher Education. All rights reserved. 8
  • Slide 9
  • THE UNDISTRIBUTED MIDDLE Another form: X is a Z. Y is a Z. Therefore, X is a Y. One other form is: If P is true, then Q is true. If R is true, then Q is true. Therefore if P is true, then R is true. 2015 McGraw-Hill Higher Education. All rights reserved. 9
  • Slide 10
  • THE UNDISTRIBUTED MIDDLE Here is an example of that last form. If Bill wins the lottery, then hell be happy. If Bill buys a new car, then hell be happy. Therefore, if Bill wins the lottery, then hell buy a new car. If P is true, then Q is true. If R is true, then Q is true. Therefore if P is true, then R is true. 2015 McGraw-Hill Higher Education. All rights reserved. 10
  • Slide 11
  • FALLACIES OF LANGUAGE Some fallacies related to discussions in Chapter 3 on ambiguity are up next: Equivocation Amphiboly Composition Division 2015 McGraw-Hill Higher Education. All rights reserved. 11
  • Slide 12
  • The Fallacies of Equivocation and Amphiboly 2015 McGraw-Hill Higher Education. All rights reserved. 12
  • Slide 13
  • EQUIVOCATION Equivocation occurs in this argument because the word bank is ambiguous and used in two different senses: All banks are alongside rivers, and the place where I keep my money is a bank. Therefore the place where I keep my money is alongside a river. 2015 McGraw-Hill Higher Education. All rights reserved. 13
  • Slide 14
  • AMPHIBOLY This occurs when the structure of a sentence makes the sentence ambiguous. If you want to take the motor out of the car, Ill sell it to you cheap. The pronoun it may refer to the car or to the motor. It isnt clear which. It would be a fallacy to conclude one way or the other, without more information. 2015 McGraw-Hill Higher Education. All rights reserved. 14
  • Slide 15
  • The Fallacies of Composition and Division 2015 McGraw-Hill Higher Education. All rights reserved. 15
  • Slide 16
  • COMPOSITION A fallacy that happens when a speaker or writer assumes that what is true of a group of things taken individually must also be true of those same things taken collectively; or assumes that what is true of the parts of a thing must be true of the thing itself. This building is made from rectangular bricks; therefore, it must be rectangular. 2015 McGraw-Hill Higher Education. All rights reserved. 16
  • Slide 17
  • Confusing Fallacies: Composition versus Hasty Generalization Composition Jumping from a fact about individual members of a collection to a fact about the collection. Hasty Generalization Jumping from a fact about an individual member of a collection to a conclusion about every individual member of the collection. 2015 McGraw-Hill Higher Education. All rights reserved. 17
  • Slide 18
  • Confusing Fallacies: Composition versus Hasty Generalization Composition The Senators are all large. Therefore, the senate is large. Hasty Generalization Senator Brown is overweight. Therefore, all the senators are overweight. 2015 McGraw-Hill Higher Education. All rights reserved. 18
  • Slide 19
  • DIVISION A fallacy that happens when a speaker or writer assumes that what is true of a group of things taken individually must also be true of those same things taken collectively; or assumes that what is true of the parts of a thing must be true of the thing itself. This building is circular; therefore, it must be made from circular bricks. 2015 McGraw-Hill Higher Education. All rights reserved. 19
  • Slide 20
  • Confusing Fallacies: Division versus Accident Division Jumping from a fact about the members of a collection taken collectively to a conclusion about the members taken individually. Accident Jumping from a generalization about every individual member of a collection to a conclusion about this or that member of the collection. 2015 McGraw-Hill Higher Education. All rights reserved. 20
  • Slide 21
  • Confusing Fallacies: Division versus Accident Division This is a large senate. Therefore, each senator is large. Accident Senators are wealthy. Therefore, Senator Brown is wealthy. 2015 McGraw-Hill Higher Education. All rights reserved. 21
  • Slide 22
  • CONFUSING EXPLANATIONS WITH EXCUSES The fallacy of presuming that when someone explains how or why something happened, he or she is either excusing or justifying what happened. I heard on the History Channel about how the weak German economy after World War I contributed to the rise of Adolf Hitler. Whats that about? Why would the History Channel try to excuse the Germans? 2015 McGraw-Hill Higher Education. All rights reserved. 22
  • Slide 23
  • CONFUSING CONTRARIES AND CONTRADICTORIES Contradictory claims are claims that cannot have the same truth value. Contrary claims are claims that cannot both be true but can both be false. 2015 McGraw-Hill Higher Education. All rights reserved. 23 VISITOR: I understand that all the fish in this pond are carp. CURATOR: No, quite the opposite, in fact. VISITOR: What? No carp?
  • Slide 24
  • CONSISTENCY AND INCONSISTENCY An individual is inconsistent if he/she says two things that cant both be true. I think taxes should not be raised. [One year later]: I think taxes should be raised. The fact that an individual has been inconsistent doesnt mean that his/her present belief is false. 2015 McGraw-Hill Higher Education. All rights reserved. 24
  • Slide 25
  • Flip-flopping is no reason for thinking that the persons current belief is defective. An inconsistent position cannot of course be accepted, but one of the beliefs of an inconsistent person may well be, depending on its merits. And dont forget, if both beliefs are contraries, they might both be false. 2015 McGraw-Hill Higher Education. All rights reserved. 25 CONSISTENCY AND INCONSISTENCY
  • Slide 26
  • Miscalculating probabilities Independent Events Gamblers Fallacy Overlooking Prior Probabilities Overlooking False Positives 2015 McGraw-Hill Higher Education. All rights reserved. 26
  • Slide 27
  • MISCALCULATING PROBABILITIES Bills chances of becoming a professional football player are about 1 in 1,000, and Hals chances of becoming a professional hockey player are about 1 in 5,000. So the chance of both of them becoming professionals in their respective sports is 1 in 6,000. NOPE. The two events, Hal becoming a hockey player and Bill becoming a football player, are independent. 2015 McGraw-Hill Higher Education. All rights reserved. 27
  • Slide 28
  • INDEPENDENT EVENTS One independent event cannot affect the outcome of another independent event. To calculate the probability that independent events both occur, we multiply their individual probabilities. The probability of both Hal and Bill becoming pro is 1/1000 times 1/5000 which is 1/5,000,000. 2015 McGraw-Hill Higher Education. All rights reserved. 28
  • Slide 29
  • THE GAMBLERS FALLACY When we dont realize that independent events really are independent, that past performance of an independent event will not influence a subsequent performance of that kind of event, Then we are at risk of committing the Gambler Fallacy. 2015 McGraw-Hill Higher Education. All rights reserved. 29
  • Slide 30
  • THE GAMBLERS FALLACY Remember, independent events do not affect one anothers outcome. Example: No matter how many times a fair coin is flipped, no matter how many times Tails has been the outcome of those flips, the probability that the next flip will show Heads is still exactly . And, for that matter, there is the same probability that it will come up Ta