Oh, hello. [SOUND] Welcome to the second unit of our
course. In the first unit, you learned how to
listen to what someone was saying, or read what they were writing, and find the
argument in their words. You learned what arguments are, what
their parts are, and what they're used for.
In this second unit and the third unit that follows it, you're going to learn
how to evaluate arguments. In the second unit, we're going to focus
on deductive arguments. And in the third unit, you're going to
focus on inductive arguments. So before we go any farther, I first need
to explain the difference between deductive and inductive arguments.
A deductive argument is an argument that's presented as being valid.
Now, remember what it is for an argument to be valid.
An argument is valid, just in case, there's no possible way for it's
conclusion to be false, when all of it's premises are true.
See, I just got a new pet clown fish, Nimo.
Now, maybe you don't know much about clown fish anatomy.
But I'm going to try to persuade you right now that clown fish have gills.
Here's an argument that I can give you for the conclusion that clown fish have
gills. Well, catfish have gills and goldfish
have gills, and sharks have gills. Therefore, clown fish have gills.
Now, is that argument valid? No, it's not.
It's not valid because it's possible for the premises to be true even when the
conclusion is false. It could be that catfish, and sharks, and
goldfish all have gills even though clownfish don't.
But now suppose I give you a different argument, for the conclusion that clown
fish have gills. Here's how this different argument goes.
All fish have gills, clown fish are a kind of fish.
Therefore, clown fish have gills. Now, that argument is valid.
There's no possible way for the premises of that argument to be true if the
conclusion is false. I just gave you an example of a valid
argument. But a valid argument is not necessarily a
deductive argument. And the deductive argument is not
necessarily a valid argument. A deductive argument is an argument
that's presented as being valid. A deductive argument is successful only
if it's valid. And in this second unit of the course,
we're going to learn how to evaluate deductive arguments for validity.
How to figure out whether or not they are valid.
So, I just gave an example of an argument and we figured out that it was valid.
We figured out that it was valid by noticing that there was no possible way
for the premises to be true when the conclusion was false.
So, if we could figure out that, that argument was valid, why do we need to do
anything more for the second unit of the course?
It looks like we can just look at an argument and figure out whether or not
it's valid. What's there to learn in this second
unit? Well, what we're going to do in this
second unit is learn some rules that we can use for figuring out when particular
arguments are valid. Now, why are these rules good to learn?
There are three reasons why it's good to learn these rules.
The first reason is that, sometimes using these rules can help you figure out
whether a particular argument is valid. You see in some cases, it's not obvious
just from looking at a particular argument that it is valid.
Consider the following example. No fish have wings.
All birds have wings. All animals with gills are fish.
Therefore, no birds have gills. Is that argument valid?
Take a moment to think about it. It's not obvious is it?
Well, if you knew the rules for evaluating arguments for validity,
you'd be able to use those rules to figure out that, that argument is valid.
So, that's one reason why it's good to learn these rules.
Sometimes they help you to figure out whether a particular argument is valid.
But there's another reason why it's good to learn these rules.
Even if they don't help you to figure out whether a particular argument is valid,
they can help you to figure out why a particular argument is valid.
So, in the example that we just considered about birds and fish, it turns
out that there's a reason why that argument is valid.
And the reason has to do with the meanings of the words, all and know.
Some of the rules that we'll learn for evaluating the validity of arguments, are
rules that tell us the meanings of the words all and know as they occur in that
argument. And so those rules explain why the
argument about the birds and the fish is valid.
And they explain it in terms of the meaning of the words all and know that
occur in that argument. So that's the second reason why it's good
to learn these rules. Sometimes these rules help us to figure
out why a particular argument is valid. There's a third reason why it's good to
learn these rules for evaluating validity.
And that is that, once we know these rules, evaluating the validity of a
deductive argument becomes a lot more fun.
To illustrate, here's some people trying to evaluate the validity of deductive
arguments without using these rules. Now, here's some people trying to
evaluate the validity of deductive arguments using these rules.
See what I mean? The second group is obviously having more
fun than the first. So, for those three reasons, it's good to
learn these rules. They can help you figure out when an
argument is valid. They can help you figure out why an
argument is valid. And, it can be a lot fun to use them.
So now, time to learn the rules.