Lecture 4-1 - Intro to Deductive Arguments (7:11)

Oh, hello. 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 t hat'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 wi th 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 "no" 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 "no" 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.