Fundamentals: Soundness

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Fundamentals: Soundness발음듣기

(intro music) Hi, I'm Aaron Ancell.발음듣기

I'm a graduate student at Duke University, and in this video I'm going to tell you about soundness, an important notion that philosophers use to evaluate arguments.발음듣기

Let's start by looking back at validity.발음듣기

You should already know what a valid argument is.발음듣기

If you don't, I encourage you to watch the video on validity before watching the rest of this video.발음듣기

As you learned in the video on validity, an argument is valid if it is impossible for all of the premises to be true while its conclusion is false.발음듣기

For example, the following is a valid argument.발음듣기

Premise (1): All cats are purple.발음듣기

Premise (2): Everything that is purple is a person.발음듣기

Conclusion: Therefore, all cats are people.발음듣기

This argument is valid, because it is impossible for the premises to be true while the conclusion is false.발음듣기

If all cats were purple, and all purple things were people, then all cats would be people.발음듣기

Of course, not all cats are purple, and not all purple things are people.발음듣기

So even though this argument is valid, it's not really informative.발음듣기

It does not establish the truth of its conclusion, since the premises are obviously false.발음듣기

Since the goal of an argument is usually to show that some conclusion is true, we usually want arguments that are more than just valid.발음듣기

This is where the notion of soundness comes in.발음듣기

Soundness is a technical notion in philosophy.발음듣기

What philosophers mean by "sound" is a bit different than what people ordinarily mean when they say things like "that was sound advice," or "she demonstrated sound judgement in making that decision."발음듣기

In philosophy, soundness, like validity, applies only to deductive arguments.발음듣기

In order to be sound, an argument must meet two requirements.발음듣기

First, the argument must be valid. All invalid arguments are unsound.발음듣기

Second, the premises of the argument must all be true.발음듣기

Any argument that has even a single false premise is unsound.발음듣기

To be sound, an argument must meet both requirements.발음듣기

Let's go back to the example with the purple cats.발음듣기

Is this argument sound? Let's check.발음듣기

The argument is valid, so it meets the first requirement.발음듣기

But it definitely does not meet the second requirement, since not all of its premises are true.발음듣기

In fact, both the premises are false.발음듣기

But not every unsound argument has false premises. Consider another example.발음듣기

Premise (1): All dead parrots are dead.발음듣기

Premise (2): Parrots are not frogs.발음듣기

Conclusion: Therefore, frogs exist. Both premises of this argument are true, so this argument satisfies the second requirement for being a sound argument.발음듣기

However, it doesn't satisfy the first requirement, because the argument is invalid.발음듣기

The conclusion does not follow from the premises.발음듣기

So this is an unsound argument, even though all the premises are true.발음듣기

Note that the conclusion is also true.발음듣기

But that doesn't matter. It's still an unsound argument.발음듣기

Here's another example.발음듣기

Premise (1): Ostriches cannot fly.발음듣기

Premise (2): All insects wear top hats.발음듣기

Conclusion: Therefore, ostriches are insects.발음듣기

This argument fails to meet both requirements.발음듣기

It isn't valid, and the second premise is false.발음듣기

So this argument is definitely unsound.발음듣기

Now, you might ask, "why should I care "whether an argument is sound?"발음듣기

The reason is that if we know that an argument is sound, then we know that the conclusion of that argument must be true.발음듣기

There is no way that an argument can meet both requirements for soundness and have a false conclusion.발음듣기

To meet the first requirement, an argument must be valid.발음듣기

And by definition, a valid argument is one where the conclusion cannot be false if the premises are true.발음듣기

And to meet the second requirement, the premises of the argument must all be true.발음듣기

Putting the requirements for soundness together, we can say that a sound argument is one where the conclusion cannot be false if the premises are true, and where the premises are all true.발음듣기

This shows that the conclusion of a sound argument cannot be false. It has to be true.발음듣기

Sound arguments are very useful.발음듣기

They enable us to establish that things are true.발음듣기

Let's finish off by looking at an example.발음듣기

Premise (1): Whales do not have fur.발음듣기

Premise (2): Whales are mammals.발음듣기

Conclusion: Therefore, not all mammals have fur. This argument is valid.발음듣기

If the premises are true, then the conclusion must also be true.발음듣기

And the premises are true, so this is a sound argument, and the conclusion must be true. Give it a try.발음듣기

See if you can write a sound argument of your own.발음듣기

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