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Oh, hello. Welcome to the second unit of
our course. In the first unit, you learned

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how to listen to what someone was saying,
or read what they were writing, and find

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the argument in their words. You learned
what arguments are, what their parts are,

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and what they're used for. In this second
unit and the third unit that follows it,

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you're going to learn how to evaluate
arguments. In the second unit, we're going

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to focus on deductive arguments. And in
the third unit, you're going to focus on

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inductive arguments. So before we go any
farther, I first need to explain the

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difference between deductive and inductive
arguments. A deductive argument is an

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argument that's presented as being valid.
Now, remember what it is for an argument

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to be valid. An argument is valid, just in
case, there's no possible way for it's

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conclusion to be false, when all of it's
premises are true. See, I just got a new

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pet clown fish, Nimo. Now, maybe you don't
know much about clown fish anatomy. But

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I'm going to try to persuade you right now
that clown fish have gills. Here's an

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argument that I can give you for the
conclusion that clown fish have gills.

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Well, catfish have gills and goldfish have
gills, and sharks have gills. Therefore,

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clown fish have gills. Now, is that
argument valid? No, it's not. It's not

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valid because it's possible for the
premises to be true even when the

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conclusion is false. It could be that
catfish, and sharks, and goldfish all have

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gills even though clownfish don't. But now
suppose I give you a different argument,

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for the conclusion that clown fish have
gills. Here's how this different argument

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goes. All fish have gills, clown fish are
a kind of fish. Therefore, clown fish have

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gills. Now, that argument is valid.
There's no possible way for the premises

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of that argument to be true if the
conclusion is false. I just gave you an

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example of a valid argument. But a valid
argument is not necessarily a deductive

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argument. And the deductive argument is
not necessarily a valid argument. A

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deductive argument is an argument t hat's
presented as being valid. A deductive

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argument is successful only if it's valid.
And in this second unit of the course,

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we're going to learn how to evaluate
deductive arguments for validity. How to

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figure out whether or not they are valid.
So, I just gave an example of an argument

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and we figured out that it was valid. We
figured out that it was valid by noticing

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that there was no possible way for the
premises to be true when the conclusion

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was false. So, if we could figure out
that, that argument was valid, why do we

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need to do anything more for the second
unit of the course? It looks like we can

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just look at an argument and figure out
whether or not it's valid. What's there to

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learn in this second unit? Well, what
we're going to do in this second unit is

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learn some rules that we can use for
figuring out when particular arguments are

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valid. Now, why are these rules good to
learn? There are three reasons why it's

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good to learn these rules. The first
reason is that, sometimes using these

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rules can help you figure out whether a
particular argument is valid. You see in

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some cases, it's not obvious just from
looking at a particular argument that it

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is valid. Consider the following example.
No fish have wings. All birds have wings.

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All animals with gills are fish.
Therefore, no birds have gills. Is that

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argument valid? Take a moment to think
about it. It's not obvious is it? Well, if

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you knew the rules for evaluating
arguments for validity, you'd be able to

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use those rules to figure out that, that
argument is valid. So, that's one reason

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why it's good to learn these rules.
Sometimes they help you to figure out

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whether a particular argument is valid.
But there's another reason why it's good

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to learn these rules. Even if they don't
help you to figure out whether a

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particular argument is valid, they can
help you to figure out why a particular

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argument is valid. So, in the example that
we just considered about birds and fish,

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it turns out that there's a reason why
that argument is valid. And the reason has

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to do wi th the meanings of the words, all
and know. Some of the rules that we'll

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learn for evaluating the validity of
arguments, are rules that tell us the

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meanings of the words "all" and "no" as they
occur in that argument. And so those rules

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explain why the argument about the birds
and the fish is valid. And they explain it

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in terms of the meaning of the words "all"
and "no" that occur in that argument. So

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that's the second reason why it's good to
learn these rules. Sometimes these rules

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help us to figure out why a particular
argument is valid. There's a third reason

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why it's good to learn these rules for
evaluating validity. And that is that,

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once we know these rules, evaluating the
validity of a deductive argument becomes a

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lot more fun. To illustrate, here's some
people trying to evaluate the validity of

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deductive arguments without using these
rules. Now, here's some people trying to

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evaluate the validity of deductive
arguments using these rules. See what I

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mean? The second group is obviously having
more fun than the first. So, for those

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three reasons, it's good to learn these
rules. They can help you figure out when

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an argument is valid. They can help you
figure out why an argument is valid. And,

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it can be a lot fun to use them. So now,
time to learn the rules.