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.