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批判性思维基础:可靠性

CRITICAL THINKING - Fundamentals: Soundness [HD]

嗨 我是Aaron Ancell
Hi, I’m Aaron Ancell.
毕业于杜克大学
I’m a graduate student at Duke University,
在这个视频中我将会告诉你有关可靠性定理
and in this video I’m going to tell you about soundness,
一个哲学家们常用来评估论证的重要概念
an important notion that philosophers use to evaluate arguments.
现在让我们回顾下有效性
Let’s start by looking back at validity.
但首先 你需要知道什么是有效性定理
You should already know what a valid argument is.
如果你不知道 我还是建议你在看该视频
If you don’t, I encourage you to watch the video on validity
之前看看关于有效性定理的科普视频
before watching the rest of this video.
就像视频中说的有效性定理那样
As you learned in the video on validity,
如果一个论证是有效的:那么它绝不可能在结论是错误的
an argument is valid if: it is impossible for all of the premises to be true
情况下其前提全是对的
while its conclusion is false.
比如 这里有一个有效性论证
For example, the following is a valid argument.
前提一:所有的猫都是紫色的
Premise (1): All cats are purple.
前提二:所有紫色的事物都是人
Premise (2): Everything that is purple is a person.
结论:所以 所有的猫都是人类
Conclusion: Therefore, all cats are people.
这个论证是有效的
This argument is valid,
因为它在前提是对的情况下结论不可能错误
because it is impossible for the premises to be true while the conclusion is false.
如果所有的猫都是紫色的 并且所有紫色的事物都是人类
If all cats were purple, and all purple things were people,
这样的话所有的猫就都是人类了
then all cats would be people.
当然 不是所有的猫都是紫色的
Of course, not all cats are purple,
而且不是所有紫色的东西都是人类
and not all purple things are people.
所以即使这个论证是有效的
So even though this argument is valid,
它也不是绝对真实的
it’s not really informative.
当前提明显错误时
It does not establish the truth of its conclusion,
论证无法得出正确的结论
since the premises are obviously false.
因为论证的目的通常是证明某一结论正确
Since the goal of an argument is usually to show that some conclusion is true,
所以仅有有效性是不够的
we usually want arguments that are more than just valid.
于是 可靠性定理就来了
This is where the notion of soundness comes in.
可靠性定理是一种哲学概念
Soundness is a technical notion in philosophy.
哲学家们所说的”sound”
What philosophers mean by “sound”
不同于人们在日常中的用法
is a bit different than what people ordinarily mean
当他们说“多么可靠的一个建议啊”或者
when they say things like “that was sound advice,” or
“她在做出决定时展现出可靠的判断力”
“she demonstrated sound judgement in making that decision.”
在哲学界 可靠性 就像有效性一样
In philosophy, soundness, like validity,
只服务于推理论证
applies only to deductive arguments.
为了使其可靠 一个论证必须得满足两个要求
In order to be sound, an argument must meet two requirements.
第一个 这个论证必须是有效的
First, the argument must be valid.
任何无效的论证都是不可靠的
All invalid arguments are unsound.
第二 该论证的所有前提都必须是正确的
Second, the premises of the argument must all be true.
只要有一个前提是错误的 那该论证就是不可靠的
Any argument that has even a single false premise is unsound.
所以只有满足以上所有的要求 该论证才可靠
To be sound, an argument must meet both requirements.
让我们回到那个紫色猫的例子上去
Let’s go back to the example with the purple cats.
这个论证是可靠的吗?
Is this argument sound?
推理一下
Let’s check!
这个论证是有效的 那么它就满足第一个要求
The argument is valid, so it meets the first requirement.
但是它根本不可能满足第二个要求
But it definitely does not meet the second requirement,
除非这些前提都是对的才行
since not all of its premises are true.
实际上 这里的前提全是错的
In fact, both the premises are false.
但是不是所有的不可靠性论证都有错误的前提
But not every unsound argument has false premises.
来看看其他的例子
Consider another example.
前提一:所有的躺尸鹦鹉都倒下了
Premise (1): All dead parrots are dead.
前提二:鹦鹉不是青蛙
Premise (2): Parrots are not frogs.
结论:所以 青蛙活着
Conclusion: Therefore, frogs exist.
这个论证的两个前提都是对的
Both premises of this argument are true,
所以该论证满足可靠性定理的第二个要求
so this argument satisfies the second requirement for being a sound argument.
然而 它不满足于第一个要求
However, it doesn’t satisfy the first requirement,
因为此论证是无效的
because the argument is invalid.
结论并不是通过那些前提来得出的
The conclusion does not follow from the premises.
所以这是一个不可靠定理论证
So this is an unsound argument,
即使它所有的前提都是对的
even though all the premises are true.
注意这个结论也是对的
Note that the conclusion is also true.
但是无所谓
But that doesn’t matter.
它依旧是个不可靠论证
It’s still an unsound argument.
让我们看看其他例子
Here’s another example.
前提一:鸵鸟不会飞
Premise (1): Ostriches cannot fly.
前提二:所有的昆虫都带着大礼帽
Premise (2): All insects wear top hats.
结论:所以 鸵鸟就是昆虫
Conclusion: Therefore, ostriches are insects.
这个论证不满足任何一个要求
This argument fails to meet both requirements.
它无效 并且第二个前提还是错的
It isn’t valid, and the second premise is false.
所以这个论证肯定是不可靠的
So this argument is definitely unsound.
“那么” 你可能会问“为什么我要管'这个论证是否可靠'这件事呢?”
“Now,” you might ask, “why should I care “whether an argument is sound?”
原因就是 如果我们知道这个论证是可靠的
The reason is that if we know that an argument is sound,
那么我们就知道了该论证的结论
then we know that the conclusion of that argument
一定是对的
must be true.
当一个论证满足
There is no way that an argument can meet
可靠性定理的要求时
both requirements for soundness
结论不可能是错误的
and have a false conclusion.
为了满足第一个要求
To meet the first requirement,
一个论证必须有效
an argument must be valid.
根据定义 一个有效的论证是
And by definition, a valid argument
当前提都对时
is one where the conclusion cannot be false
结论不可能错误
if the premises are true.
而且为了满足第二个要求
And to meet the second requirement,
该论证的所有前提都必须是对的
the premises of the argument must all be true.
将可靠性定理的要求放在一起
Putting the requirements for soundness together,
可以说 一个可靠性论证
we can say that a sound argument
就是如果前提都是对的话
is one where the conclusion cannot be false
其结论不可能是错误的一种论证
if the premises are true.
而且可靠性论证的所有前提都得是对的
And where the premises are all true.
以上展示了 可靠性论证的结论不可能是错的
This shows that the conclusion of a sound argument cannot be false.
它肯定是正确的
It has to be true.
可靠性论证非常有用
Sound arguments are very useful.
能帮我们肯定某一结论为真
They enable us to establish that things are true.
让我们以一个例子来收尾吧
Let’s finish off by looking at an example.
前提一:鲸没有毛皮
Premise (1): Whales do not have fur.
前提二:鲸是哺乳动物
Premise (2): Whales are mammals.
结论:因此 不是所有的哺乳动物都有毛皮
Conclusion: Therefore, not all mammals have fur.
这个论证是有效的
This argument is valid.
如果这个前提是正确的
If the premises are true,
那么结论就一定是对的
then the conclusion must also be true.
而这里的前提都是对的 所以这是一个可靠性定理
And the premises are true, so this is a sound argument,
因此它的结论 一定是对的
and the conclusion must be true.
来试试
Give it a try.
看看你能不能自己写出一个可靠性论证来
See if you can write a sound argument of your own.

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视频概述

有关可靠性定理的讲解,但是最好先有有效性定理的基础。成为可靠性论证需要满足两个要求:1,论证具有有效性;2,论证的前提全部都是正确的。

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视频来源

https://www.youtube.com/watch?v=3P0fUHUaZcs

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