Arguments and Evidence
- 1. Arguments
- 2. Definition of ‘Argument’ • We will spend nearly the entire semester identifying, evaluating, and presenting arguments. So, we need to know what they are. • An argument is a set of sentences one of which is the conclusion and the rest of which are offered as evidence for the conclusion; these are called premises. (For us, an argument is not a quarrel between people—though those occasionally contain arguments in the sense we care about.) • So, what is evidence?
- 3. Evidence: Bad Definition One Consider some possible definitions: 1. A is evidence for B if A supports B (or proves B). This definition doesn’t really clear anything up. What does it mean for A to support B (or to prove b)?
- 4. Evidence: Bad Definition Two 2. 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 people actually reason. What could be wrong with that? Follow the link below for a brief experiment that you should try before moving on: http://www.philosophyexperiments.com/mary/Def ault.aspx
- 5. Bad Definition Two, Continued • That experiment is a variation of one first performed by Tversky and Kahneman in 1983. In their study 89% of respondents made the mistake that you probably made or luckily avoided. (Manktelow, K., Thinking and Reasoning. Psychology Press, New York, 2012. p.30) • Once you read the explanation, it’s obvious that we shouldn’t take the description of Mary as evidence that the 4th option is more likely than the 2nd option, despite the fact that almost everyone uses the description to draw that conclusion (that the 4th is more likely than the 2nd). So we need a new definition of evidence.
- 6. Evidence: Bad Definition Three • A is evidence for B if people should conclude B on the basis of A • This defines evidence in terms of the way people ought to reason (notice how it differs from Definition Two). You might not like this definition because it seems to make a value judgment. However, that’s not the problem with it—there are objectively good ways to reason and objectively bad ways; for the most part we should opt for the good over the bad. So what is the problem?
- 7. A Brief Aside: Pascal’s Wager • Blaise Pascal was a 17th century mathematician who, among other things, invented (with Pierre Fermat) probability theory to help his wealthy friends make better gambling decisions. He realized that his methods could be applied to all manner of decisions— like the decision to believe that God exists. • There are two possibilities for the world independent of you: either God exists or not. Also, there are two possibilities for you: either you can believe that God exists or not. This gives four total possibilities. They can be represented by the following simple grid. (Inside the squares are the approximate payoffs to you.)
- 8. Payoff Table God Exists God Does not Exist You 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 who rewards belief and punishes disbelief. • Notice that if God doesn’t exist, then it doesn’t much matter what you believe. However, if God does exist then one of your options is substantially better than the other. So, Pascal says, you should believe that God exists.
- 9. Bad Definition Three, Continued • Suppose Pascal’s table is roughly correct, has he given a reason to believe God exists? That is, should people conclude (believe) that God exists on the basis of the information in the table? • Has Pascal given evidence that God exists? • If you think ‘yes’ is a reasonable answer to the first question and that ‘no’ is a good answer to the second question then you don’t think that Definition Three is adequate. (Do you see why?)
- 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 and legs if you don’t believe that our company is the best to work for.” You should, if you believe the threat, say immediately that it is the best company to work for. But suppose that your boss can tell when you don’t really believe what you’re saying, and she repeats the threat. Shouldn’t you really try to believe that it’s the best company? (You should if you value your limbs.) • Of course, reasons that you should believe are not the same as evidence, since your boss has given you reason to believe but has not given you evidence that your company is the best. (In fact, if your boss makes a threat like that, you got some evidence for just the opposite conclusion.)
- 11. Good Definition? • So what is evidence? • A is evidence for B when A is true and either guarantees or makes probable that B is true. • We’ll figure out just what’s involved in this answer over the course of the term. Since arguments are presentations of evidence, by studying arguments to distinguish good ones from bad ones, we will figure out what good evidence is and what it isn’t.