0:00:02.660,0:00:08.096
The next question has probably been[br]bothering you ever since very early on in

0:00:08.096,0:00:13.874
the previous lecture. Namely, if valid[br]arguments can have false premises, then

0:00:13.874,0:00:19.154
what good are they? Sure, there's this[br]technical logician's notion of a valid

0:00:19.154,0:00:24.919
argument, but why should we care whether[br]arguments are valid if valid arguments can

0:00:24.919,0:00:30.824
be really bad? Validity might be necessary[br]for an argument to be good or at least for

0:00:30.824,0:00:35.965
a deductive argument to be good because[br]remember, there are also inductive

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arguments. But even though it's necessary,[br]it's not enough. You can have a horrible

0:00:42.407,0:00:48.260
argument but still valid. Well, the great[br]thing about validity is that when you add

0:00:48.260,0:00:54.042
true premises to a valid argument, then[br]you get something that really is valuable,

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which we're going to call a sound[br]argument. Because if you know that the

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premises are true and you also know that[br]it's not possible for the premises to be

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true and the conclusion, false, then, you[br]know, the conclusion must be true. So, in

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a sound argument, the conclusion has to be[br]true. And that is what makes it valuable

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cuz if we can get a deductive argument to[br]be sound, then you really got something.

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What you've got is a true conclusion.[br]Officially then, a sound argument is one

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where the premises are true and the[br]argument is valid. And we've got the same

0:01:35.167,0:01:40.574
combinations of truth and falsity as[br]possibilities that we had in valid

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arguments. You can have both premises and[br]conclusion are true and then, if it's

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valid, the argument is sound and if it's[br]not valid, it's not. Or you can have the

0:01:52.289,0:01:58.147
premises are true and the conclusions[br]false and then, it can't be valid. But if

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it's invalid, it's not sound. We can have[br]the premises are false and the conclusions

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true. And then if it's valid, it's not[br]sound and if it's invalid, it's not sound.

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Or we can have both the premises and the[br]conclusion are false, and then, it's not

0:02:17.014,0:02:22.784
going to be sound whether it's valid or[br]not. So, the only combination, where it

0:02:22.784,0:02:28.944
sound is when the premises are true and[br]the argument is valid and, in that case,

0:02:28.944,0:02:34.517
you know that the conclusion is true. What[br]about lack of soundness? Well, there are

0:02:34.517,0:02:39.754
two ways an argument yjay fail to be[br]sound, namely, either the argument can be

0:02:39.754,0:02:44.785
invalid or one of its premises can be[br]false. So, it's a lot easier for an

0:02:44.785,0:02:50.294
argument to be unsound. And we know that a[br]deductive argument tries to be valid and,

0:02:50.294,0:02:55.592
of course, it wants its premises to be[br]true so a deductive argument is trying to

0:02:55.592,0:03:00.582
be sound. And when it fails to be sound,[br]it's not going to be any good. And the

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next question is how can you know? If you[br]don't know whether the premises are true,

0:03:06.437,0:03:11.059
you're not going to know whether the[br]arguments sound. Well, not quite, because

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if you, if the argument's valid and you[br]know it's valid, then you don't know

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whether it's sound unless you know the[br]premises are true. But if you that the

0:03:20.552,0:03:25.237
argument is invalid, you already know it's[br]unsound, even if you don't know whether

0:03:25.237,0:03:30.046
the premises are true. So, if you think[br]about it, that shows why you want to be

0:03:30.046,0:03:34.869
able to test for validity. Because if you[br]can show the argument's invalid, then

0:03:34.869,0:03:39.669
you're going to be able to, well, I know[br]it's unsound, regardless of what you think

0:03:39.669,0:03:44.407
about whether the premises are true or[br]not. So, there's going to be some value to

0:03:44.407,0:03:49.084
validity, namely, if you can show it's[br]invalid, you're going to show it's unsound

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and that means that the deductive argument[br]didn't get what it wanted. So, validity is

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going to be necessary for soundness and[br]soundness is going to be important because

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it guarantees the truth of the conclusion,[br]and then, validity derives its value from

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the fact that if it's not valid, it's not[br]sound. Okay. Now, there's a more to say

0:04:12.814,0:04:17.227
about validity. And we'll say a lot more[br]about validity when we get to a formal

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logic in the second part of this course.[br]But for now, we're just going to stick

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with this pretty intuitive notion of[br]validity and see how we can use this

0:04:27.252,0:04:30.027
notion of validity to reconstruct[br]arguments.