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The next question has probably been 
bothering you ever since very early on in 

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the previous lecture. Namely, 
if valid arguments can have false 

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premises, then what good are they? 
Sure, there's this technical logician's 

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notion of a valid argument, but why 
should we care whether arguments are 

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valid if valid arguments can be really 
bad? 

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Validity might be necessary for an 
argument to be good or at least for a 

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deductive argument to be good because 
remember, there are also inductive 

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arguments. 
But even though it's necessary, it's not 

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enough. 
You can have a horrible argument but 

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still valid. 
Well, the great thing about validity is 

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that when you add true premises to a 
valid argument, then you get something 

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that really is valuable, which we're 
going to call a sound argument. 

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Because if you know that the premises are 
true and you also know that it's not 

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possible for the premises to be true and 
the conclusion, false, 

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then, you know, the conclusion must be 
true. 

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So, in a sound argument, 
the conclusion has to be true. 

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And that is what makes it valuable 
because if we can get a deductive 

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argument to be sound, 
then you really got something. 

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

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where the premises are true and the 
argument is valid. 

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And we've got the same combinations of 
truth and falsity as possibilities that 

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we had in valid arguments. 
You can have both premises and conclusion 

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are true and then, if it's valid, the 
argument is sound and if it's not valid, 

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it's not. 
Or you can have the premises are true and 

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the conclusions false and then, it can't 
be valid. 

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But if it's invalid, it's not sound. 
We can have the premises are false and 

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the conclusions true. 
And then if it's valid, it's not sound 

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and if it's invalid, it's not sound. Or 
we can have both the premises and the 

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conclusion are false, and then, it's not 
going to be sound whether it's valid or 

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not. 
So, the only combination, where it sound 

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is when the premises are true and the 
argument is valid and, in that case, you 

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know that the conclusion is true. 
What about lack of soundness? 

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Well, there are two ways an argument yjay 
fail to be sound, namely, 

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either the argument can be invalid or one 
of its premises can be false. 

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So, it's a lot easier for an argument to 
be unsound. 

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And we know that a deductive argument 
tries to be valid and, of course, it 

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wants its premises to be true so a 
deductive argument is trying to be sound. 

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And when it fails to be sound, it's not 
going to be any good. 

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And the next question is how can you 
know? 

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If you don't know whether the premises 
are true, you're not going to know 

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whether the arguments sound. 
Well, not quite, because if you, if the 

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argument's valid and you know it's valid, 
then you don't know whether it's sound 

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unless you know the premises are true. 
But if you that the argument is invalid, 

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you already know it's unsound, even if 
you don't know whether the premises are 

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true. 
So, if you think about it, that shows why 

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you want to be able to test for validity. 
Because if you can show the argument's 

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invalid, then you're going to be able to, 
well, I know it's unsound, regardless of 

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what you think about whether the premises 
are true or not. 

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So, there's going to be some value to 
validity, namely, if you can show it's 

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invalid, you're going to show it's 
unsound and that means that the deductive 

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argument didn't get what it wanted. 
So, validity is going to be necessary for 

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soundness and soundness is going to be 
important because it guarantees the truth 

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of the conclusion, and then, validity 
derives its value from the fact that if 

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it's not valid, it's not sound. 
Okay. 

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Now, there's a more to say about 
validity. 

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And we'll say a lot more about validity 
when we get to a formal logic in the 

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second part of this course. 
But for now, we're just going to stick 

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

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notion of validity to reconstruct 
arguments.