Arguments and Evidence


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Arguments and Evidence

  1. 1. Arguments
  2. 2. Definition of ‘Argument’• We will spend nearly the entire semesteridentifying, evaluating, and presentingarguments. So, we need to know what they are.• An argument is a set of sentences one of which isthe conclusion and the rest of which are offeredas evidence for the conclusion; these are calledpremises. (For us, an argument is not a quarrelbetween people—though those occasionallycontain arguments in the sense we care about.)• So, what is evidence?
  3. 3. Evidence: Bad Definition OneConsider some possible definitions:1. A is evidence for B if A supports B (or provesB).This definition doesn’t really clear anything up.What does it mean for A to support B (or toprove b)?
  4. 4. Evidence: Bad Definition Two2. A is evidence for B if most people would(actually) conclude B on the basis of A.This defines evidence in terms of the way peopleactually reason. What could be wrong withthat?Follow the link below for a brief experiment thatyou should try before moving on:
  5. 5. Bad Definition Two, Continued• That experiment is a variation of one first performed byTversky and Kahneman in 1983. In their study 89% ofrespondents made the mistake that you probably madeor luckily avoided. (Manktelow, K., Thinking andReasoning. Psychology Press, New York, 2012. p.30)• Once you read the explanation, it’s obvious that weshouldn’t take the description of Mary as evidence thatthe 4th option is more likely than the 2ndoption, despite the fact that almost everyone uses thedescription to draw that conclusion (that the 4th ismore likely than the 2nd). So we need a new definitionof evidence.
  6. 6. Evidence: Bad Definition Three• A is evidence for B if people should conclude B on thebasis of A• This defines evidence in terms of the way people oughtto reason (notice how it differs from Definition Two).You might not like this definition because it seems tomake a value judgment. However, that’s not theproblem with it—there are objectively good ways toreason and objectively bad ways; for the most part weshould opt for the good over the bad. So what is theproblem?
  7. 7. A Brief Aside: Pascal’s Wager• Blaise Pascal was a 17th century mathematicianwho, among other things, invented (with PierreFermat) probability theory to help his wealthy friendsmake better gambling decisions. He realized that hismethods could be applied to all manner of decisions—like the decision to believe that God exists.• There are two possibilities for the world independentof you: either God exists or not. Also, there are twopossibilities for you: either you can believe that Godexists or not. This gives four total possibilities. Theycan be represented by the following simple grid.(Inside the squares are the approximate payoffs toyou.)
  8. 8. Payoff TableGod Exists God Does not ExistYou Believe God Exists ∞ (eternal paradise) ~0 (not much loss or gain)You don’t Believe -∞ (ouch) ~0 (not much loss or gain)• These payoffs apply, of course, only to a God whorewards belief and punishes disbelief.• Notice that if God doesn’t exist, then it doesn’t muchmatter what you believe. However, if God does existthen one of your options is substantially better thanthe other. So, Pascal says, you should believe thatGod exists.
  9. 9. Bad Definition Three, Continued• Suppose Pascal’s table is roughly correct, has hegiven a reason to believe God exists? Thatis, should people conclude (believe) that Godexists on the basis of the information in thetable?• Has Pascal given evidence that God exists?• If you think ‘yes’ is a reasonable answer to thefirst question and that ‘no’ is a good answer tothe second question then you don’t think thatDefinition Three is adequate. (Do you see why?)
  10. 10. Bad Definition Three, Concluded• Here’s a simpler way of showing Definition Three is bad:Suppose your employer tells you “I’ll break your arms andlegs if you don’t believe that our company is the best towork for.” You should, if you believe the threat, sayimmediately that it is the best company to work for. Butsuppose that your boss can tell when you don’t reallybelieve what you’re saying, and she repeats the threat.Shouldn’t you really try to believe that it’s the bestcompany? (You should if you value your limbs.)• Of course, reasons that you should believe are not thesame as evidence, since your boss has given you reason tobelieve but has not given you evidence that your companyis the best. (In fact, if your boss makes a threat likethat, you got some evidence for just the oppositeconclusion.)
  11. 11. Good Definition?• So what is evidence?• A is evidence for B when A is true and eitherguarantees or makes probable that B is true.• We’ll figure out just what’s involved in thisanswer over the course of the term. Sincearguments are presentations of evidence, bystudying arguments to distinguish good onesfrom bad ones, we will figure out what goodevidence is and what it isn’t.