I ~ l l i o tSober, t Core Questions in Philosophy (6" e d . 2 0 1 2 ) . 1 Part One: apter 2 Deductive Arguments Introduction asstirnptions. Ln geometry yuu may have spent little or no lime askina whether the axioms arc trilc. Not so [or the philonnphlcd arguments I discus in thir bwt. W'c'll want tn see wlictlier the prcllltses are plausible. We'll also want to see whether the l>rr~nises, if they were true, would prwidc a nxson for thinltlng the conclusion is true as well. 1'11 pose these two questions again and again. Good Arguments I now want to talk about different kinds of "good arguments? What does "good" mean? A good argument Is iwtionaJIy persuasive it glves you a substantial reason to think the conciusion is true, Adverfisers and politicians sametimes use arguments that trtck people into believing what they say These arguments sometimes persuade people, but they don't always pmnde gomi reasons. A good argument should have true premises; if the premises are false, how could they give you good reasons to believe the conclusion? Rut mom is required than this. In the following argument, the premise is true, but it doesn't provide you R good reason to think that the conclusion is true: Chapter Outline Good Arguments Testing for Invalidity Circularity, or Begglng the Question Deductive Validity Defined *Validityty" Is a Technical Term Tmth "True for W Logical Form Inralldity Wishful Thfnking Self-TalflUing Prophesies Argument8 Philosophy involves constructing and evaluating arguments. In this respect, philosophy is no different from any other rational activitp-mathemat~cims do this, as do economists, physicists, and people in everyday hfe. The distincblve thing about philosophy isn't that philosophers construct and evaluate arguments: what is distinctive is the k~ndsof questron.5 those arguments aim to answer. In the previotls chapter, I talked about what makes a question philosophical. The goal in this chapter is to develop some techniques that can be used to tell whether an argument Is good or bad. Arguments Grass is green Roses are red What is wrong here is that the premises are irrelevant to the conclusian. A good argument should contain true premises, but it should also cite premises that are related in the right way to the conc~usion.The truth of the premises should give you a reason to think that the conclusion is true. The thee types o f u g dargument" that I'll now describe differ in what relationship their premises and wndusions have to each other. Good arguments can be divided into two categories, and one of those categories can be divided into two more: Good Armrnents / - \ D&uctivelv Valid Not Deductivelv Valid / \ Inductivelv Abductivcly Strong Strong An argument divides into two parts: the premises and the ~onclusion.The premlses and the conclusion are statements; each is expresed by a declarative sentence. Each is either true or false. When people argue that a given statement i s true, they try toprwide reasons for th~nkingthis. The reasons are the premises of their arpment; premises are assumptions. The statement to be established is the argument's conclusion. In high school geometry, you talked about axioms and theorems. Axioms are assumptlans (prcrnises); the theorem (the canclus>on)is what 1s supposed to follow from those I'll treat the three categories (deductivelyd i d , inductivelystrong, and abductively strong) as mueudIy actusiw. Jf an w e n t belongs to one category, it can't belong to any of the others. At the end of Chapter 3, I'11 modlfy this dassification slightly. Y w may have heard some of this hrmino!ogy before. Deduction i s what you do in a mathematical proof. Induction involves sampling from a pop~~latton to decide what its characteristics are. "Abduction" may be a less familiar term. It has nothing lo do wlth - .... ' . . . l . . . * . . . , . , " .......+ . . . , . Chaptar +: Deductlv* Argumsntr . . . . . I . . . . . I . . . . . . . . . . . . . . . . . . . . . . . Wdnapping. The word was invented by the great dneteenth-centuryAmerican philosopher Charla Sanders P e w Philosopherssometimes use the longer lab4 "inference to the best explanation'' to describe what Pe~rwmeant by abduction. fI consider deduction in this chapter, induction and abduction in the next The god in ea&-casc is to describe some ofthe considerationsthat are rdwantto deciding whether an argument is good or bad. Deductive Validity Defined The first type of good argument consists of ones that are *deductivety valid.? Here are two eampIes of this type of argument: AU fish swim. AU shark are fish. AU partid- have mass. All electrons are particles. M sharks swim. All eIectmns haw mass. In these arguments. the premises are the statements above the horizontal line; the condusion b the statement below. These arguments say that the premises are true and that, M o r e , the conclusion also 18 true. Hwe is what deductive validity means: A deductive@valid argument is an argument that has the following property: IF in '3 I ..... Part One: ln!mduction Logical Form What makes an argument dcduct~vclyvalid? Tbe thmc example arguments described so Iar have diflercnt suhject matters. Thc first is about fish, the second is about particles, and the third ir about plants. Although they are ahout different things, they have the rnme slnlctrm. The structural property [hat they have in common is called their 'logical lorrnP Think of each argument as the result of substituting terms into the forlaving skeleton: All Bs are O All As are Bs All As are Q Thls is the logical form of the three arguments given. You can think of A, B, and C as blanh into which terms may be substituted. Take a mlnute to see how the arguments just stated can be obtained from the above skeleton by substitution-by *filling In the blanks." An argument is valid or invalid solely because of the logical form it has. The subject matter of the argument Is irrelevant. Since the three example arguments have the same logical form, they are all valid or 111 invalid. They have the same logcal form. ao thgr are in the same boat. As already mentioned, they are valid Indeed, each and every one of the millions of arguments you can construct by substitution into the above skeleton is valid as well. pmnises were true, its c~nctusionwould have to be true. I've capitalized the word IF: I'd print it in bright colors ifI auld because it is important not to forget thls wo-letter word. A valid argument need not how true premises. What Is required is that the mndusion would have to be true IF the premises were true. Take a minute to look at these two arguments Conrlnce yourself that they are deductively valid. Validityn Is a Technical Term What philosophers and logicians mean by 'Galid" doesn't have much in common with what we mean by *validmin ordinary English. In everyday life, we say that a statement is Palid* if it is plausible or true. The technical use of the term that 1just explained dfffers f m ordinary usage la two ways. Flrst, wtnever say that a statement or an idea is valid or invalid. Valldity is a property of arguments and of arguments only. Second, an argument can be valid even if the statements it contains arewtldly implausible. Avalid argument can have false premfses and a false conclusion. Here is an example: All pIants haw minds. A1I ladders are plants, All Iadders have minds. Invalidity The definition of validity tells you what a deductively invalid argument will be like, If there is even t h e smallest possibflity that the conclusion could be false when the premies are true, then the w p m h n t 1s deductiwly invalid The ladder argument is valid, although all the statements it contains are false. L the reverse situation possible? Can an argument be invaIid, even t h o u ~ haU the statements if contains are true? The answer is yes. Here's an example: Emeralds are gre@n. Lemons we yellm The premise is true, tord so Is the conduslon. Sa why isn't the argument deductiwly valid? The definition ofvalidity says that the premises In a d i d argument must prwide an nbsolwteguarantcc that the conclusion is tme. Rut the fact that emeralds are green doesn't guarantee that lemons must be yellow. The color af lemons tsni' entalled by the fact that emeralds are green. Validity conccrns the mlatronship of premises m conclusion, not the queshon of whether the premises and the condusIon each happen to be true. Sometimes it isn't so obvious that an argument is t d d . T f i e a h e example is prep blatant-the premisehas nnthing to do with the conclusion. But what do yw thinkof the following at.gumcnt? Is it valid or not? - Ckmpter n: D d u ~ I I v eArguments IfJones stands In the heavy rain wlthout an umbrella, then Jones will get wet. 1011- Is wet. Jones was standing In the heavy rdn without an umbrella. Imagine that all t h m .of the statements in this argument are hue Ima@nethat rones is now standing before you w d n g wet and that Jonesjust came in from the rain. Even ifall the statements in this argument are true, this argument Is still Inrelid. It is just l$c the argument about emeralds and lemons. Although the premises and the conclusion happen to be true, the premises don't guarantee that the conclusion must be true. How can we see this more d d y ?I said before that all arguments that have the same logical form are in the same boat Thtf means that if the argument about Jones is Invalid, so is each and evwy argument that bas the same form. Let's begin by isolating the argument's logical f m . Here it i s 15 Part One: lniroductiun the conclusion is false. If you can find even one rottell apple of this we,you arc finished. If there is evcn une argument with this property, thm every argument ofthat form is invalid When an argument has true premiscs and a false co~~cIuslon, it 1s quife ohlous that the truth of the premises dwsn'tpmntee that the conclusion must be true. The premises can't be guaranteeing this, as the conclus~onIS false. But this tells you something general. It tells you that each and every argument of this form will be such that the prcmises don't guarantee the truth nf the conclusion. So Ear, I t e presented someexamples oiargumentr I'veexplained that a valid argument needn't have true statements In it and that an argument composed solely of true statements needn't b e e h d . This should make you wonder whether tberc Is any connectron at all between the question of whether an argument is valid and the question of whether the premlses and conclusion are true There is a connection. It is iilustrated by the following table. Ifan argument is valid, it can exhibit three of the four following combinations in which the premises are either all true or not all true and the conclusion is either true or false: h i r e s at do and Q stand for in thIs argument skeleton? You can subslltute any statement (declarative sentence) you please for these letters to obtain an argument with this logfcal form Notice that the letters in this skeleton differ in their function from the letters in the previous skeleton. There, A, $ and C are blanks into which terms denoting kinds of thing5 [Wsh,'' "electmn%'etc.) can be suhsiituted Anyhow, we now have the loeicsl form of the " argument about Jones.If it is invalld, so are all arguments that have the same logical f o m This means that iffhere is even one a r m e n t that bas t h i s l .,o n i c a l h in which the ~ m iaes are true and the condusion false, then the argument form is i n d i d . This will mean that the initial argument about Jones is itrvalid as well, Here i s an argument that has the same lopica1 form as the argument about Jones that settles the question: If Sam Iives in Wiswnsin, then Sam lives in the United States. Sam lives in the United States. Sam Iives in Wisconsin. The premises of this.argument are true, but the conclusion, J mure you, is false. The Sam I'm talking about lives In Georgia Ali true Not all true True Possible Possible False ImpossiMc Possible Conclusion This table indicates that a d i d argument m ' t have ew premises and a false conclusion. However, the faa that an argument is valid leaves open which of the other three cells the argument occupies. What can be said of an imlld a r p e n t ? If an argument is invalid, are any of the fmr combinations Impossible? 1 leave thrs to you to figure out by consulting the definition of didiLy. When you find an invalid argument, you may want to ask if the argument can be repaired Is there anything that can be done to an i n d i d argument to turn it into an argument that is valid? There ia By adding premises, p u can turn a deductijely i n d i d argument into a valid one. Consider the following argument: Smith liws in the United States. Smith liws in Wwnsin. Thls is invalid, but it can be made valid by adding a premise: Testing for Invalidity -- So here's a strategyto we if you want to knowwhether an argument it invalid Fint, ignore the Smith Iives in the United StatEveryone who lives In the United States lives in 1Visconsin. argument'ssubject matter and isolate 'the logical form (the "skeletonn)of the argument. Second, see if you can invent an argument that has this logicdl form in which the premises are true and Smith lives in Wisconsin. Chapter I: Osductlve Ar(umnts .................................................................. . . . . Notice that the conclusion now follows from the premises. The trouble is that the second premise is false. In the preceding pair of arguments, r i n g the defect of invalidity just substitutes one problem for another; instead of having to criticize an argument for being invalid you now have m d t i d z e an argument for having a premise that isn't true. The following argument pair is different. Here you can repair the defect of invalidity and obtain a perfectly fine argument. Notice first that the following argument isn't deductiwly valid: Smith lives in Wisconsin. Smith lives inthe United States. The argument can be repaired, however, by adding a premise: Smith lives in Wisconsin. Everyone who l i e s in Wisconsin lives in the United State. Smith lives in the United States. Thfs argument is valid and has true premises as well. You can see from these two pain of arguments that invalidity is easy to fix. Just add premises. What is harder is to add premises that not only make the argument valid, but that are true as well. Thls idea will come up repeatedly when I discuss various philosophical arguments. I will sometimes claim an argument is invalid. When this happens, you should ask yourself whether the argument can be repaired. Often the price of making the argument valid (by adding a premise) is that you have to supply a new premise that you thirik is false. In making this addition, you are trading one defect (invalidity) for another (false premises). If you can't repair the argument so that it is both valid and has all m e premises, then you should consider the possibility that there is something fundamentally flawed about the whole line of argument On the other hand, sometimes an invalid argument can be replaced by a vahd one merely by supplying a true premise that maybe you neglected to mention because it is so obvious. In this case, the defect in the original argument isn't fundamental. So far I've emphasized two questions that we will want to ask about arguments: 1. Is the argument deductivelyvalid? 2. Are all the premises true? If the answer to both questions is p the conclusion of the argument must be true. Arguments are tools We use them to do things. When the goal is rational persuasion. a good argument will provide a good reason to think that the conclusion is m e , If an argumem is deductively valid and has true premises, is that sufficient to make the argument good?To see why validity and true premises aren't enough, consider the followingargument: Lemons are yellow. Lemons are yellmv. Here the conclusion merely repeats what the premise asserts. This argument is valid and the premise is true. But there is something defective about this argument. What is it? rl part one: Introduction If/then statements are called conditionals. Cond~tionalstatements have other state. ments as components. For example, the statement "If pigs fly, then grass IS green" is a statement ofthe form "If P, then Q," whcre P and Q are themselves statements. In the statement "If P, then Q," P is called the antecedent and Q ir called the consequent. A conditional doesn't say that its antecedent is true; the statement "If Joe drinks arsenic, then )oe will die" does not say that Joe drinks arsenic. And a condltional doesn't say that i!s consequent is true; "If there is a nuclear war, then Wash~ngton will be attacked" doesn't say that Washington will be attacked. Conditionals can be rewritten without changing what they Say. Consider the state ment "Ifyou l ~ v eiq Wisconsin, then you live in the United States." Th~si s equ~valent in meaiing to "If you don't live in the United States, then you don't l~wein Wisconsin." The conditional 'If P, then Q is equivalent to "If not-Q, then not-P," no matter what P and Q happen to be. Here's a piece of terminology: The statement "If not.Q, then not-P' is the contrapositive ofthe conditional "ICP, then Q." A conditional and its contrapositive are equivalent. Consider the following two conditionals: "If P, then Q and "If Q, then P" Are they equivalent? That is, do they mean the same thing? The answer is no. 'If you live in Wisconsin,then you live in the United States" is true, but "Ifyou live in the United States, then you live in Wisconsin" is false. These two iflthen statements can't mean the same thing, because one is true while the other is false. "If Q, then P* is termed the converse of the conditional "If P,then Q." A conditional and its converse are not equivalent. Circularity, or Begging the Question The previous argument is circular: it begs the question. Supposc you didn't already have an opinion as to whether lemons are yellow. The above argument wouldn't help you resole your uncertainty. The argument would he useless in this regard. Good arguments are tools that help answer questions about whether their conclusions are true. A good argument should give you a reason to accept the conclusion if you dodt already believe the conclusion is true. So h i d e s che&ng to see if an argument is deductively valid and has true premises, you should also see if the argument begs the question. You'll notice in what I've just said that I am using the exprewion "beggmg the question" to name a defect in an argument. Unfortunately, the phrase is often used now to simply mean that a question is betng asked You'll hear this usage on the evening news-- ?he recession has not i m p d . This begs the question of whcther guvetnment action can make things better." As with the term "validity:' the term " b a i n g the question" is used by philosopherswith a special, technical meaning one that doesn't coincide with ordinary usage. Truth One other idea needs clearing up before I leave the topic of deductive validity, You'll notice that the ddmition of validity makes use ofthe concept of truth. What is truth? -- Chaptar 5: Deductive Rrgumsnts Part One: Introdudian ......._....._..........I...........,.........*.,......-............-... There are deep philosophical questions here, most of which I'll skirt. My goal is to describe the concept of truth I use in this book It is beyond the scope of this book to defend this choice or to fully develop its implications. To begin with, whether a statement is true is an entirely diierent question from whether you or anybody happens to believe it. Whether someone believes the statement *The Rocky Mountalna are in North America" is a psychoPOgica1 question. If beings with minds had never popdated the earth, no one would have thought about the location of this mountain range. But this dwsn't affect the question of whether the statement is true. There can be truths that no one believes. Symrnetrjcally, there can be propositions that everyone thinks are m e , but that aren't. Them can be beliefs that aren't tnre. To understand what makes an argument questron.begglng, it rs useful to exarnlne some examples. Suppose you were trylng to convlnce someone that God exlsts. The argument you give for th~nkingthat t h ~ sIS true is that the h b l e says that there IS a God. Would t h ~ argument s connnce someone who d~dn'talready believe that there rs a God? Probably not Anyone who doubts that there IS a God probably doesn't th~nk that everythng the Bible says 1 5true Here's a second example Someone IS very susp~c~ows about the rel~abll~ty of consumer magazines You try to convlnce h ~ mthat Consumer Reports is rel~ableby panting out that Consumer Repolas ranks ~tselfvery highly In an art~cleevaluating the rellab~lityof consumer magazines. Probably your argument wlll fa11to convlnce. In these two examples, 1dent1Sythe prernlses and conclusion In each argument Then descrrbe what ~tIS about the argument that makes it question-begg~ng. When I say that a certain sentence has the property of being true, what am I saying? For example, when E say that the English sentence *TheRocky Mountains ate in North A m e r i d is true, am I attributing some mysterious property to the sentence? Not really. All I'm saying is that the world is the way the scntence says It is. When I say that the sentence is true, all I'm saying is that the Rocky Mountains are in North America. So in a way,the concept of truth 1s often 'redundant? Sometimes when I use the concept of truth, I could say the same thing without using that concept. In high school English, your teacher might have t d d you to avoid redundancy. If you hand In an essay containing the sentence "Osmr is an unmarried bachel~r,"the essay might come back with "unmarried crossed out and the marginal comment "avoid redundancy* The word "unmarried" is redundant because "Oscnr is an unmarried bachelor" means exactly the same thing as "Oscar~sa ba~helor.~ Adding the word *unmarrieC 1s to spill useless ink. The Redundancy Theory of Truth claims that the word "truemIS redundant rn just this sense. "lt is true that the Rockies are in North America" says exactly what the sehtence "The Rockies are in North America" asserts. This helps show why truth isn't a mysteriom property If you believe a statement FI you also belleve that P is true. So,if you have any belle5 about rhe world at all, you should be qulkc cornfortable applying the concept of truth to those beliefs. U T r ~for e Me" You'll see from these remarks that the expression "It is true for me" can be dangerously rnislead~ng.Sometimes sayng that astatemen? istrue4for you" just means that you bdieve it. If that rs what you want to say, just use the ward *belief" and leave truth out of it. Howwer, there is a more controvers~alIdea that might be involved here. Sometimespeople use tlie ewpresqlon "true for me*to express the Idea that each of us makes our own reality and that the beliefs we have Eonstitute rhat reality. I'll asrume this is a mistake. My concept of truth assumes a fundamental division between the way things mlly are and the way they may seem to be m this or that indiuidual. Thia is whst I meant in Chapter I bydistinyishing the objective realm and the subjective realm. Wishful Thinking Closely relatcd to this distinction behueen objective and subjectiw is a piece of advice: We should m i d wtrhful thznking. Most of the things we believe arenl made true by our believing them. That the Rockies are m North America is a fact that is rndependent of w r thought and language. We don't bring this geographic fact into being by thfnldng or talking in the way we dn. Self-Fulfilling Prophesies In saying hi$,I'm not denyingthat the thoughts we haw often affect the world outside the mind. If I think to myseII, =I can't hit a basebalr this may have the effect that I do badly in the batter's box; here my believlng something has the effect that t h bellef is made m e . This is the idea of a "self-hlfillingprophesjr Notice how this causalchain works: Thmq$l I believr that 1won't hit the baseball. - Adion I swing toe high. - Truth 1 don't hit the basehall. My believing a proposition causes m action, which has the effect of makingthe proposition trw. I have no problem mth the idea that various statements may be caused to be true by individuals thinkingthoughts to themselves. What I deny s that the mere act of thlnldng, unconnected with action or some other causal pathway, can make statements true in the world outside the mind. I'm rejecting the idea that the world Is arranged so that it spontaneously conforms to the ideas we happen to entertain. Part On@:Iniroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Later in thh book 111 investigate whether there are any exceptions to this principle that says that we should avoid wishful thinking. Maybe there are some statements that become true just because we think they are. Here are some philosophical claim we'll dongdm Mathematical statements and ddinitlons are made true by our regarding them as r . snch: for example, "a + 3 = 5" 1s true just because we choose to deflne our termhology ('a: "+: &.I in the waywe do (Chapter4). S:me statements about the contents of my own mind (for example, 1am in pain") are made true just by believing they are true (Chapter 13). Ethical statements are true Justbecause God,sadety, or some individual agent thinks they are [Chapter 31). I'm mentioning these philosophical claims here without tlpping hand as t o whether Ithink any of them is plausible. If any of them were correct, they would be exceptions to the pattern I've just described. For the moment, though, I'm merev noting that belief and truth are generallyvery separate questions. 7. What doe5 ~tmean to 5ay that an argument IS "ctrcular,"that ~t"begs the question') Conrtrvct an example ofan arp;urnent o f t h ~ type r dlllerent Corn the ones pres~ntedIn tha~chapter 8 What does i t mean to r a y that truth IS oblectlv?, not sublcct~veJ Problems for Further Thought 1. The R~dundancyThew of Truth may seem plausible as an account of what the following sentence means: [t is true that the 'Rockies are in North America. Does it work as well as an explanation ofwhat the lollowing sentence means? Some statements that are [rue have not been formulated yet 2 Consider the following argument: I release an otherwise unsupporled apple fmm my hand a few feet from the earth's surface. The apple falls to earth. Is thrs argument dedudlvely val1d3What is the logical form oftha argumenl' I. *-2. 3. (Here 1s a problem that was drawn to my attemt~onby R~chardBehllng ) In this chapker, I sa~dthat each argument has a rrngte logical form This IS the skeleton Into wh~chterms can be subst~tuted ta obtaln the argument What I s a d is an oversirnplAcation.A g~ve"argument can be obtained from many logical forms For example, consider the follow~ngargument When is a statement or idea valid) (a tnck question) Define what it means to say that an argument is deductively valid. 3, Invent an example ofa valid argument that has false premises and a true conclusion. Invent an example of an invalid argument that has true premises and a true conclusion. 4. Can a statement be a premise in one argument and a conclusion in another? If-youthink so, give an example. 5. Which of the following argument forms is valid? Which is invalid? For each of the invalid ones, construct an example of an argument with that form in which the premises are true and the con- Fred l i e s In California. If Fred l~vesin tal~fornia,then Fred lives In the United Stales Fred lives m the Un~tedYates This argument can be obtained from both ofthe Followingskeletons by subst~tut~on: clwsion false, IFP, then Q IFP, then Q Q P la) (b) Q P Argument form (a) is valld, but (b) is mvalid The argument about Fred 1s valid If P, then Q If P, then Q Here's the problem: Use the concept of log~calform to define when an argument isualld, and when it Is inual~d,w~thoutfall~ngInto the trap of thlnklng that each argument has eractly one logical lorm. Not. P N0t.Q P-1 4. In this chapter, E claimed that there are "objective truths." Do you agree? Construct an argument in which you try to demonstrate that such things exist, or that they do not IId ) NO!-Q N0t.P For the argument forms you th~nkare Yallac~ous, Invent names 'or these fallacres by using the vocabula7 about cond1:lonals presented In the box on page 18. 6 A sign on a store says, "No shws, no senrice " Does t h ~ smean that will be served' lf you wear shoes, then you MySearchLab Connections M. Usten. Explore. Read.Vlr~tYySearchL~bfor add~tionalreadrqgs, videos, podcasts, research tools, and more. complete your study ofth1.j chapter by complet~ngthe toplc-specific and chapter earn assessments available at w w w . m y M ~ h l d . w m . WATCH AND LISTEN @WATCH 1. Listen W h the Natlon @<LISTEN : A H w t o Gulden-fPlk d . m this interview with NPR, Profirsor Simon BIatkbum dis~urserthe dcbatt Between objdctivlsts and rubJcGlivirts about trutkthmughout history and t h y . Who1 dour Blackbrrm think a wrong w ~ t hths subjenivisr porition? Do you tkrnk that here is alirolutc truth or that eoch pfion detcmines who: ir "truefor him or hcr"7 RESEARCH AND EXPLORE #CDIPLORE 1. 2, Research deductive vatldity Research oblwtive truth