(intro music)
I'm Geoff Pynn. I teach at
Northern Illinois University,
and this is an introduction to critical thinking.
In this lesson, we're gonna talk about three things.
First, what is critical thinking?
Second, what is an argument?
And third, what's the difference between
deductive and ampliative arguments?
Okay, so what is critical thinking?
Well, fundamentally, critical thinking
is about making sure that you have
good reasons for your beliefs.
What does that mean?
So suppose that you and your friend
are talking about who's gonna be at tonight's party.
And she says to you, quite confidently,
"Monty won't be at the party."
You're not sure whether or not to believe her,
so it would be natural for you to follow up
by asking, "Why do you think so?"
And there are a lot of different things
that she might say in response.
We're gonna talk about three
possible answers she could give.
First, she might say, "I can't stand him,
and I want to have a good time."
Second, she might say, "Well, he's really shy,
and he rarely goes to parties."
And third, she might say,
"He's in Beijing, and it's impossible
to get here from Beijing in an afternoon."
The first response that she gives you
does not give you a good reason to believe
that Monty won't be at the party.
The second reason, though, is a good reason
to believe that Monty won't be at the party.
If he's really shy and rarely goes to parties,
then it's probable that he won't be at tonight's party.
Similarly, the third reason also gives you a good
reason to believe that Monty won't be at the party.
If he's in Beijing, and it's impossible to get here
from Beijing in an afternoon, then it's guaranteed
that he won't be at the party.
And when you notice things like that,
when you distinguish between good
and bad reasons for believing something,
you're exercising your critical thinking skills.
So critical thinking is making sure
we have good reasons for our beliefs,
and so one of the essential skills that you learn
when you're studying critical thinking is how
to distinguish good reasons for believing something
from bad reasons for believing something.
Now, it's worth saying something about
how I'm using the term "good" here.
I'm not using it to indicate anything
having to do with morality or ethics.
So it's not morally right or morally good
to believe something on the basis of good reasons.
Similarly, it's not morally wrong, or evil, or wicked
to believe something on the basis of a bad reason.
Rather, here, what it is to say that a reason is good
is closely tied to the notion of truth.
So a good reason for a belief is one
that makes it probable, that is, it's one
that makes the belief likely to be true.
The very best reasons for a belief
make it certain, they guarantee it.
So why does this matter?
Well, the reason that critical thinking
is important is because, since we're rational,
we want our beliefs to be true.
Rational people want to have true beliefs,
and they want not to have false beliefs.
And the best way to be rational in this way
is to form beliefs only when you
find good reasons for them.
Okay, that leads us to our second question:
What is an argument?
Well, an argument is a set of statements that together
comprise a reason for a further statement.
So, for example, we can consider one
of your friend's responses before as an argument.
She's given you two statements,
"Monty's really shy" and "Monty rarely goes to parties,"
which together comprise a reason for believing
that Monty won't be at the party.
The statements that are the reason,
we call the argument's premises.
So "Monty's really shy" is premise one,
"Monty rarely goes to parties" is premise two,
and the statement that those premises give you
reason to believe, we call the argument's conclusion.
A good argument is one in which the premises
give you a good reason for the conclusion, that is,
the premises make the conclusion likely to be true.
In that case, we say that the argument
supports the conclusion.
Good arguments support their conclusions,
and bad arguments don't support their conclusions.
So a key part of critical thinking is learning
to evaluate arguments to determine
whether or not they're good or bad,
that is, whether or not their premises
support their conclusions.
The red argument is the first response
that she gave, two premises,
"I can't stand Monty" and "I want to have a good time."
And the conclusion is "Monty won't be at the party."
And the third argument, which we'll put in purple,
consisted also of two premises,
"Monty's in Beijing" and "He can't get from Beijing
to the party in time, so he won't be at the party."
Now, as I indicated before, the first argument
is not good, while the purple argument is good.
And here I can explain a little bit more about why.
If you consider what the red argument's premises say,
that your friend can't stand Monty,
and she wants to have a good time,
and think about their relationship
to the conclusion of the argument,
you'll see that those statements don't make
that conclusion any more likely to be true.
The fact that your friend can't stand Monty
and wants to have a good time doesn't do anything
to make it more likely that Monty won't be there.
It's simply unrelated to the conclusion.
In the purple argument, though, the premises,
if they're true, they guarantee the conclusion is true.
So they make it very probable.
The truth of the premises guarantees the truth
of the conclusion, and so in the purple argument,
the premises do support the conclusion.
Now, it's worth pointing out that the red argument,
though it's bad as it stands, could be made
a good argument with the addition
of some background premise.
So, for example, if you found out
that your friend was the person who decided
who was going to be invited to the party,
then the fact that she can't stand Monty
and wants to have a good time
would give you a good reason to believe
that Monty won't be at the party,
because it would give you reason
to believe that she didn't invite him.
But as it stands, the argument is not good.
Those two premises considered in themselves
give you no reason to believe
that Monty won't be at the party.
Okay, our last topic is to distinguish
two different types of arguments.
So I'm gonna put up here, on the left,
the orange argument, which is the
second response that your friend gave,
"Monty's really shy" and "He rarely goes to parties."
On the right we'll put the purple argument,
"Monty's in Beijing" and
"He can't get from Beijing to the party in time."
Both of them have the same conclusion,
"Monty won't be at the party."
Now, as I said before, both of these
are good arguments, they both do
give you reason to believe the conclusion,
i.e., both of them have premises
which support the conclusion,
but there's an important difference
between the two arguments that I want to point out.
If you consider the purple argument,
and think about what those premises say,
you'll notice that if those premises are true,
if Monty's in Beijing, and can't get from Beijing
to the party in time, then it must be true
that Monty won't be at the party.
Those premises guarantee the conclusion.
In such an argument, where the premises
guarantee the truth of the conclusion,
we call the argument deductive.
In a deductive argument, given the premises,
the conclusion must be true.
Just thinking about the information
in the premises in a deductive argument
gives you all you need to deduce the conclusion.
If you look at the orange argument, though,
you'll notice that that's not the case.
In the orange argument, even if those premises
are true, the conclusion might still be false.
Even given that Monty is really shy
and rarely goes to parties, it's still possible
that he'll get over his shyness and suspend
his policy of rarely going to parties,
and unexpectedly show up.
It's unlikely, but it's possible.
So the truth of the premises in the orange argument
does not guarantee the truth of the conclusion.
Arguments like this, we call ampliative.
In an ampliative argument, the truth of the premises
makes the conclusion probable but doesn't guarantee it.
Now, as I said, both of the arguments are good.
Ampliative arguments can often be
very good arguments, they're just not deductive.
The premises don't guarantee the truth of the conclusion.
Now, when you're evaluating an argument,
it can be important to know whether or not
the argument is supposed to be deductive
or supposed to be merely ampliative.
If an argument is supposed to be deductive,
but careful consideration of the argument
reveals that in fact the premises
don't guarantee the truth of the conclusion,
if the conclusion could be false even though
the premises are true, that's often a good reason
to reject the argument as a bad argument.
Whereas in an ampliative argument,
to notice that the truth of the premises
doesn't guarantee the truth of the conclusion,
is simply to notice that it's an ampliative argument.
If you were to object to the orange argument
by pointing out that, still, the conclusion
could be false, you'd really be missing the point.
In an ampliative argument, it's taken for granted
that the conclusion is not guaranteed by the premises.
Rather, what an ampliative argument is doing
is giving you reasons to think
that the conclusion is probable.
So knowing what type of argument an argument is
is essential to knowing which tools to use
to evaluate whether or not it's a good argument.
And we'll talk quite a bit more
about different tools for evaluating both ampliative
and deductive arguments in future lessons.
Okay, so summing up this lesson.
Critical thinking is making sure
that we have good reasons for our beliefs,
where we understand a good reason as one
that makes the belief probable, or likely to be true.
An argument is a set of statements,
which we call premises, that together comprise
a reason for another statement,
which we call the argument's conclusion.
And in a good argument, the premises
support their conclusions, that is,
the premises give you a good reason for believing
the conclusion, because they make it probable.
A deductive argument is one where the conclusion
is guaranteed by the premises. If the premises are true,
then the conclusion must be true.
An ampliative argument is one where the premises
don't guarantee the conclusion,
but they do make it probable.
So they can still provide you
with good reason for believing the conclusion.
Okay, so that ends this introductory lesson.