Hi! I'm Jeff Pynn, and I teach philosophy[br]at Northern Illinois University.
In my earlier Introduction to Critical[br]Thinking video, I described the difference
between deductive arguments and ampliative[br]arguments.
In the next few videos, I'll talk a bit[br]more about each type of argument.
Let's start with deductive arguments.
An argument is a set of statements, called[br]its premises, that are meant to give you
a reason to believe some further statement[br]called the argument's conclusion.
In some arguments, the premises are meant[br]to guarantee that the conclusion is true.
Arguments like this are called deductive[br]arguments.
A good deductive argument can give you a[br]very good reason
for believing its conclusion.
After all, it guarantees that its [br]conclusion is true.
But not all deductive arguments are good,[br]and so there are several things to think
about when deciding whether to believe the[br]conclusion of a deductive argument.
A good deductive argument really does[br]guarantee its conclusion.
Part of what this means is that its[br]impossible for the premises to be true
while the conclusion is false.
When this is the case, we say that the[br]argument is valid.
Now this is a special, technical use of[br]the word "valid."
In ordinary life, we often use this word[br]to mean something like good, cogent, or
reasonable.
Like if you're disagreeing with someone[br]about something, and they respond to a
claim you make by saying something that[br]seems pretty reasonable to you,
you might say, "Well, I guess you have[br]a valid point."
Though that's what the word often means[br]in ordinary life,
it's not what the word means here.
When philosophers say that an argument is[br]valid, they always mean this very
specific thing: that if the premises are[br]true, the conclusion must also be true.
There are several other Wi-Phi videos that[br]discuss this notion of validity
in more detail.
To say that an argument is valid is to say[br]something about the relationship between
the premises and the conclusion.
Namely, that if the premises are true, the[br]conclusion must also be true.
But it's not to say that its premises or[br]conclusion are true.
Consider, for example, this argument.
Premise 1: Beyonce was born in Paris.
Premise 2: Everybody who was born in Paris[br]loves cheese.
Conclusion: Therefore, Beyonce loves[br]cheese.
Those premises are false. Beyonce was born[br]in Houston, and I'm willing to bet that at
least some people born in Paris hate[br]cheese.
Still, it's a valid argument.
If the premises were true, then the[br]conclusion would have to be true.
But because the premises are false, this[br]argument doesn't give you a good reason to
believe its conclusion, even though it's[br]valid.
Philosophers call a valid argument with[br]true premises "sound."
Like the word "valid," the word "sound" is[br]term with various meanings in ordinary
life, and it can be used to describe some[br]claim as reasonable or compelling.
But when philosophers describe an argument[br]as sound, they always mean this very
specific thing: that it's valid, and that[br]its premises are in fact true.
Here's a pretty boring sound argument.
Premise 1: Beyonce was born in Houston.
Premise 2: Everybody who was born in[br]Houston was born in Texas.
Conclusion: Therefore, Beyonce was born in[br]Texas.
For more discussion of the concept of a[br]sound argument,
see Aaron Ancell's Wi-Phi video entitled[br]"Soundness."
So, before deciding whether to believe the[br]conclusion of a deductive argument,
you need to determine whether the argument[br]is sound.
And this, in turn, requires determining[br]whether the argument is valid,
and whether its premises are true.
Well, how do you tell whether an argument[br]is valid?
Sometimes, it's just obvious. But often,[br]it's not so obvious.
One way to figure out whether an argument[br]is valid
is to see if you can think of a[br]counterexample to it.
A counterexample is a case, either real or[br]imaginary,
where the argument's premises are true,[br]but the conclusion is false.
So, for example, consider this argument.
Premise 1: Classical musicians appreciate[br]opera.
Premise 2: Beyonce is a pop star, not a[br]classical musician.
Conclusion: Therefore, Beyonce doesn't[br]appreciate opera.
Now, suppose that Beyonce's been listening[br]to opera since she was a little girl,
and loves Mozart's Don Giovanni.
Well, then she'd appreciate opera.
The conclusion would be false, even though[br]the premises would still be true.
It would still be true that classical[br]musicians appreciate opera,
and that Beyonce is a pop star, not a[br]classical musician.
This counterexample shows that the[br]argument isn't valid,
and so that even if premises are true, the[br]argument doesn't provide you with a reason
to believe its conclusion.
There are other, more formal techniques[br]for figuring out whether an argument is
valid, which we'll hopefully be able to[br]discuss in future videos.
Now, if you don't know whether the[br]premises of an argument are true,
then even if the argument really is sound,[br]it doesn't give you a good reason to
believe its conclusion.
When you know that an argument is valid,[br]but you don't know whether its premises
are true, the argument gives you, at best,[br]a conditional reason
to accept its conclusion.
If you learn that its premises are true,[br]then you'll have to accept its conclusion.
So, how do you tell whether an argument's[br]premises are true?
Well, this isn't the kind of thing logic[br]or philosophy can give you much help with.
To figure out whether an argument's[br]premises are true,
you need to do some research.
This is one reason why being a good[br]critical thinker requires more
than just logical ability.
It also takes a lot of real world,[br]empirical knowledge.
Unless you know enough to know whether an[br]argument's premises are true, then even if
you're a really brilliant logician and[br]know that the argument is valid,
it doesn't give you reason to believe its[br]conclusion.
The more you know, the better you'll be[br]able to evaluate deductive arguments.
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