批判性思维 (2).pdf

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1、 Time to look more closely at argumentsthe kind that actually show something(unlike the red herrings and emotional appeals and other fallacies we are going to be talking about in a moment).ARGUMENTS:GENERAL FEATURES To repeat,an argument consists of two parts.One part,the premise,is intended to prov

2、ide a reason for accepting the second part,the conclusion.This statement is not an argument:God exists.It s just a statement.Likewise,this is not an argument:God exists.That s as plain as the nose on your face.It s just a slightly more emphatic statement.Nor is this an argument:God exists,and if you

3、 don t believe it,you will go to hell.It just tries to scare us into believing God exists.Students will learn to.1.Recognize complications regarding premises and conclusions 2.Distinguish between deductive and inductive arguments3.Understand the standards for valid-ity,soundness,strength,and weak-ne

4、ss in arguments 4.Assess an argument with an unstated premise5.Distinguish between ethos,pathos,and logos6.Identify a balance-of-considerations argument7.Identify an inference to the best explanation(IBE)8.Use techniques for understanding arguments 2 Two Kinds of Reasoning 37moo38286_ch02_037-068.in

5、dd 37moo38286_ch02_037-068.indd 3712/9/10 1:31 PM12/9/10 1:31 PM38 CHAPTER 2:TWO KINDS OF REASONING Also not an argument:I think God exists,because I was raised a Baptist.Yes,it looks a bit like an argument,but it isn t.It merely explains why I believe in God.On the other hand,this is an argument:Go

6、d exists because something had to cause the universe.The difference between this and the earlier examples?This example has a prem-ise(“something had to cause the universe”)and a conclusion(“God exists”).As we explained in Chapter 1(see pages 810),an argument always has two parts:a premise part and a

7、 conclusion part.The premise part is intended to give a reason for accepting the conclusion part.This probably seems fairly straightforward,but one or two complications are worth noting.Conclusions Used as Premises The same statement can be the conclusion of one argument and a premise in another arg

8、ument:Premise:The brakes aren t working,the engine burns oil,the trans-mission needs work,and the car is hard to start.Conclusion 1:The car has outlived its usefulness.Conclusion 2:We should get a new car.In this example,the statement“The car has outlived its usefulness”is the conclusion of one argu

9、ment,and it is also a premise in the argument that we should get a new car.Clearly,if a premise in an argument is uncertain or controversial or has been challenged,you might want to defend itthat is,argue that it is true.When you do,the premise becomes the conclusion of a new argument.How-ever,every

10、 chain of reasoning must begin somewhere.If we ask a speaker to defend each premise with a further argument,and each premise in that argu-ment with a further argument,and so on and so on,we eventually fi nd our-selves being unreasonable,much like four-year-olds who keep asking,“Why?”until they becom

11、e exasperating.If we ask a speaker why he thinks the car has outlived its usefulness,he may mention that the car is hard to start.If we ask him why he thinks the car is hard to start,he probably won t know what to say.Unstated Premises and Conclusions Another complication is that arguments can conta

12、in unstated premises.For example,Premise:You can t check out books from the library without an ID.Conclusion:Bill won t be able to check out any books.The unstated premise must be that Bill has no ID.An argument can even have an unstated conclusion:Example:The political party that best refl ects mai

13、nstream opin-ion will win the presidency,and the Republican Party best refl ects mainstream opinion.If a person said this,he or she would be implying that the Republican Party will win the presidency;that would be the unstated conclusion of the argument.moo38286_ch02_037-068.indd 38moo38286_ch02_037

14、-068.indd 3812/9/10 1:31 PM12/9/10 1:31 PM TWO KINDS OF ARGUMENTS39 Unstated premises are common in real life because sometimes they seem too obvious to need mentioning.The argument“the car is beyond fi xing,so we should get rid of it”actually has an unstated premise to the effect that we should get

15、 rid of any car that is beyond fi xing;but this may seem so obvious to us that we don t bother stating it.Unstated conclusions also are not uncommon,though they are less com-mon than unstated premises.We ll return to this subject in a moment.TWO KINDS OF ARGUMENTS Good arguments come in two varietie

16、s:deductive demonstrations and induc-tive supporting arguments.Deductive Arguments The premise(or premises)of a good deductive argument,if true,proves or dem-onstrates (these being the same thing for our purposes)its conclusion.How-ever,there is more to this than meets the eye,and we must begin with

17、 the fundamental concept of deductive logic,validity.An argument is valid if it isnt possible for the premise to be true and the conclusion false.This may sound complicated,but it really isn t.An example of a valid argument will help:Premise:Jimmy Carter was president immediately before Bill Clinton

18、,and George W.Bush was president immediately after Bill Clinton.Conclusion:Jimmy Carter was president before George W.Bush.In DepthConclusion IndicatorsWhen the words in the following list are used in arguments,they usually indicate that a prem-ise has just been offered and that a conclusion is abou

19、t to be presented.(The three dots repre-sent the claim that is the conclusion.)Thus.Consequently.Therefore.So.Hence.Accordingly.This shows that.This implies that.This suggests that.This proves that.Example:Stacy drives a Porsche.This suggests that either she is rich or her parents are.The conclusion

20、 isEither she is rich or her parents are.The premise isStacy drives a Porsche.moo38286_ch02_037-068.indd 39moo38286_ch02_037-068.indd 3912/9/10 1:31 PM12/9/10 1:31 PM40CHAPTER 2:TWO KINDS OF REASONING As you can see,it s impossible for this premise to be true and this conclusion to be false.So the a

21、rgument is valid.However,you may have noticed that the premise contains a mistake.Jimmy Carter was not president immediately before Bill Clinton.George H.W.Bush was president immediately before Bill Clinton.Nevertheless,even though the premise of the preceding argument is not true,the argument is st

22、ill valid,because it isn t possible for the premise to be true and the conclusion false.Another way to say this:If the premise were true,the conclusion could not be falseand that s what“valid”means.Now,when the premise of a valid argument is true,there is a word for it.In that case,the argument is s

23、aid to be sound.Here is an example of a sound argument:Premise:Bill Clinton is taller than George W.Bush,and Jimmy Carter is shorter than George W.Bush.Conclusion:Therefore,Bill Clinton is taller than Jimmy Carter.This argument is sound because it is valid and the premise is true.As you can see,if a

24、n argument is sound,then its conclusion has been demonstrated.Inductive Arguments Again,the premise of a good deductive argument,if true,demonstrates that the conclusion is true.This brings us to the second kind of argument,the inductive argument.The premises of good inductive arguments don t demon-

25、strate their conclusions;they support them.For example:A woman has been found murdered.The husband is known to have threatened her repeatedly.In DepthPremise IndicatorsWhen the words in the following list are used in arguments,they generally introduce premises.They often occur just after a conclusio

26、n has been given.A premise would replace the three dots in an actual argument.Since.For.In view of.This is implied by.Example:Either Stacy is rich or her parents are,since she drives a Porsche.The premise is the claim that Stacy drives a Porsche;the conclusion is the claim that either Stacy is rich

27、or her parents are.moo38286_ch02_037-068.indd 40moo38286_ch02_037-068.indd 4012/9/10 1:31 PM12/9/10 1:31 PM TWO KINDS OF ARGUMENTS41That fact certainly does not demonstrate that the woman s husband murdered her.By itself,the fact barely even supports that conclusion.But it does support it slightly.I

28、t raises the probability slightly that the husband was the murderer.Certainly the investigators should question the husband closely if they learn he repeatedly threatened his wife before she died.If you are thinking that support is a matter of degrees and that it can vary from just a little to a who

29、le lot,you are right.If,say,the husband s fi ngerprints had been found on the murder weapon,that fact would offer much better sup-port for the conclusion that the husband was the murderer.That is,it would make it likelier that the woman s murderer was her husband.Inductive arguments are thus better

30、or worse on a scale,depending on how much support their premises provide for the conclusion.Logicians have a technical word to describe this situation.The more support the premise of an inductive argument provides for the conclusion,the stronger the argument;the less support it provides,the weaker t

31、he argument.Put another way,the more likely the premise makes the conclusion,the stronger the argument;and the less likely,the weaker the argument.Discovering that the man repeatedly threatened his wife(that s the premise)raises the probability slightly that it is he who was the murderer(that s the

32、conclusion).By comparison,discovering that his fi ngerprints are on the murder weapon raises the probability by a much larger jump:It is the stronger of the two arguments.Real LifeAbe Lincoln Knew His LogicValidity and Soundness in the Lincoln-Douglas DebatesHeres Abraham Lincoln speaking in the fif

33、th Lincoln-Douglas debate:I state in syllogistic form the argument:Nothing in the Constitution.can destroy a right dis-tinctly and expressly affirmed in the Constitution.The right of property in a slave is distinctly and expressly affirmed in the Constitution.Therefore,nothing in the Constitution ca

34、n destroy the right of property in a slave.Lincoln goes on to say:There is a fault in the argument,but the fault is not in the reasoning;but the falsehood in fact is a fault of the premises.I believe that the right of property in a slave is not distinctly and expressly affirmed in the Constitution.I

35、n other words,the argument is valid,Lincoln says,but unsound and thus not a good argument.Syllogisms,by the way,are covered in Chapter 8.moo38286_ch02_037-068.indd 41moo38286_ch02_037-068.indd 4112/9/10 1:31 PM12/9/10 1:31 PM42CHAPTER 2:TWO KINDS OF REASONING Many instructors use the word“strong”in

36、an absolute sense to denote only those arguments whose premise gives the conclusion better than a 50-50 chance of being true.In this book,however,we use“strong”and“weak”in a comparative sense.Given two arguments for the same conclusion,the one whose premise makes the conclusion more likely is the st

37、ronger argument,and the other is the weaker.These are a lot of concepts for you to remember,but you shouldn t be surprised if your instructor asks you to do so.To make this task easier,let s summarize everything to this point.Again,the two basic types of arguments are(1)deductive demonstrations and(

38、2)inductive supporting arguments.When we reason deductively,we try to prove or demonstrate a conclusion.A deductive argument is said to be valid if it isn t possible for the prem-ise to be true and the conclusion false.Further,if the premise of a valid argument is in fact true,the argument is said t

39、o be sound.The conclusion of a sound argument has been proved or demonstrated.When we reason inductively,we try to support a conclusion.Inductive arguments are“stronger”or“weaker”depending on how much support the premise provides for the conclusion;that is,depending on how likely the premise makes t

40、he conclusion.If ever,oh ever,a Wiz there was The Wizard of Oz is one because because because because because.Because of the wonderful things he does!“Because”is sometimes followed by a premise,and sometimes by a phrase that refers to a cause.Which is it here?ANSWER:“.the wonderful things he does”is

41、 evidence he is a wonderful wizard;it is a premise.moo38286_ch02_037-068.indd 42moo38286_ch02_037-068.indd 4212/9/10 1:31 PM12/9/10 1:31 PM DEDUCTION,INDUCTION,AND UNSTATED PREMISES 43 Beyond a Reasonable Doubt In common law,the highest standard of proof is proof“beyond a reasonable doubt.”If you ar

42、e a juror in a criminal trial,evidence will be presented to the courtfacts that the interested parties consider relevant to the crime.Addi-tionally,the prosecutor and counsel for the defense will offer arguments con-necting the evidence to(or disconnecting it from)the guilt or innocence of the defen

43、dant.When the jury is asked to return a verdict,the judge will tell the jury that the defendant must be found not guilty unless the evidence proves guilt beyond a reasonable doubt.Proof beyond a reasonable doubt actually is a somewhat lower standard than deductive demonstration.The latter correspond

44、s more to what,in ordinary En glish,might be expressed by the phrase“beyond possible doubt.”Recall that in logic,a proposition has been demonstrated when it has been shown to be the conclusion of a sound argumentan argument,that is,in which(1)all premises are true,and(2)it is impossible for the prem

45、ises to be true and for the conclusion to be false.In this sense,many propositions people describe as having been demon-strated or proved,such as that smoking causes lung cancer or that the DNA found at a crime scene was the defendant s,have not actually been proved in our sense of the word.So,in re

46、al life,when people say something has been demonstrated,they may well be speaking“informally.”They may not mean that something is the conclusion of a sound argument.However,when wethe authorssay that something has been demonstrated,that is exactly what we mean.DEDUCTION,INDUCTION,AND UNSTATED PREMIS

47、ES Somebody announces,“Rain is on its way.”Somebody else asks how he knows.He says,“There s a south wind.”Is the speaker trying to demonstrate or prove rain is coming?Probably not.His thinking,spelled out,is probably something like this:Stated premise:The wind is from the south.Unstated premise:Arou

48、nd here,south winds are usually followed by rain.Conclusion:There will be rain.In other words,the speaker was merely trying to show that rain was a good possibility.Notice,though,that the unstated premise in the argument could have been a universal statement to the effect that a south wind always is

49、 followed by rain at this particular location,in which case the argument would be deductive:Stated premise:The wind is from the south.Unstated premise:Around here,a south wind is always followed by rain.Conclusion:Rain is coming.Spelled out this way,the speaker s thinking is deductive:It isn t possi

50、ble for the premises to be true and the conclusion to be false.So one might wonder abstractly what the speaker intendedan inductive argument that supports the belief that rain is coming,or a deductive demonstration.There is,perhaps,no way to be certain short of asking the speaker some-thing like,“Ar