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.