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