And it's all too easy to reach the false conclusion that the danger is past.
人们很容易做出错误的判断，认为危险已经过去了。
If it's could true premises and a false conclusion it couldn't be valid. Otherwise it could be.
如果有真前提和假结论，它就不可能有效，否则就可能有效。
One of the interesting things about valid arguments as you'll see is that they can have false conclusion.
关于有效论点的一件很有意思的事，就像你们看到他们可能有错误的结论。
And you might ask yourself, why should an argument that has a false conclusion be an useful argument. Mind at good argument.
你们可能问自己，为什么一个有错误的结论的论点，应该是一个有效的论点，这是好论点。
It's the possibility of combination of true premises and false conclusion that is ruled out not just the actuality of the combination.
真前提和假结论结合的可能性，被排除了，不只是结合的实际性。
That's why truth preservingness is useful in whatever the situation is, even if the conclusion is false.
这就是为什么真理保留是有用的,不管情境如何，即使结论为假。
They are both invalid there are possible situation where the premise is true and the conclusion false they are not good arguments.
他们都是无效的，有可能的情境，前提为真结论为假，他们都不是好论点。
Sentence statements can be true or false, so premises can be true or false, conclusion can be true or false but arguments can't be.
句子论述可能或真或假，所以前提前提可能或真或假，结论可能或真或假，但是论点不能。
If one premise if false that doesn't guarantee the truth of the conclusion does it?
如果一个前提为假那就不能保证，结论的真理，对吗？
So if we see the conclusion of a valid argument is false what do you know? A little bit longer.
如果我们看到有效论点的结论为假，你们知道什么？有一点长。
If we can show the conclusion of a valid argument - put your hand up-if we can show the conclusion of a valid argument is false what do we know?
如果我们可以展示有效论点的结论-，举手，如果我们可以展示，有效论点的结论为假，我们知道什么？
That's right. So if you have a valid argument a deductively valid argument in which the conclusion is false then you know one of the premises must be false.
对，所以如果论点有效，一个演绎的有效论点中结论是假的，你就知道前提中有一项是假的。
And the reason for that because an argument is valid if and only if there are no possible situation in which all its premises are true and its conclusion false.
原因是因为论点有效，如果且仅如果没有可能情况，所有的前提都为真结论为假。
But what we are doing here is lets assume the conclusion is false and lets if these all can be true together.
但是我们做的是假设结论为假，如果这些都为真。
Prof. : uh no because the conclusion is that they will need help, don't forget the premise is maybe false.
教授：呃，不是，因为结论是，他们会需要帮助，别忘了前提可能会错。
Because there is no possible situation in which the premises are true and the conclusion false.
因为没有可能情境，其中前提为真结论为假。
No sorry there are no possible situation where the premises are true and the conclusion false. Is there?
不是，没有可能情境，前提为真结论为假，不是吗？
Okay so the only thing that's ruled out in arguments being valid is the possibility, not the actuality, of the combination of the premises being true and the conclusion false.
好，唯一能排除论点有效性的是，是可能性，不是实际性，而是真前提，和假结论的结合。
But let me ask again, is there any possible situation where that premise is true and that conclusion is false.
但是我再问问，有没有可能，前提为真结论为假。
If the premises could be true together with the conclusion being false then the argument is invalid.
如果前提可能为真，同时结论为假，论点有效。
So this world the premises are true true and false and the conclusion is true. Is that alright?
这个世界上前提为真真假，结论为假，这对吗？
I'm sorry, that conclusion in with the premises they set up a situation where there is no possible situation where all the premises are true and the conclusion false.
抱歉，把结论放到前提里，他们建立起情境，不可能的情境，所有的前提都为真，结论为假。
And I am saying actually the discovery that the valid argument is that the conclusion in which is false can be hugely important.
我说实际上发现那是有效论点，结论为假，可能很重要。
So have a look at that. Is there a s situation in which the premises could be true and the conclusion false.
所以看看吧，有没有一个情境，前提为真结论为假。
Is this a situation in which the premises are both true and the conclusion false? No.
这个情况中前提都为真，结论为假？不。
The only thing that's ruled out in arguments being valid is the possibility of the combination of the premises being true and the conclusion false.
唯一排除论点有效性的是,真前提和假结论,结合的可能性。
Here one premise is true and the other is false and the conclusion is false. That seems to be valid as well.
一个前提为真，另一个为假，结论为假，这看起来有效。
So it's a sufficient condition for an argument being valid that there is no possible situation where the premises are true and the conclusion false. And that's a necessary condition too.
所以这对于论点有效性是充分条件，没有可能前提为真，结论为假，这是必要条件。
So now we've got the set consisting of the premises just as they are plus we are pretending the conclusion is false.
现在我们有前提组成的,加上我们假装结论为假。
How about the premises are both false and the conclusion false, in fact that can also be valid.
前提都为假，结论为假，实际上可能也有效。
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