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批判性思维基础:演绎论证

CRITICAL THINKING - Fundamentals: Deductive Arguments

大家好 我是杰夫·林恩
Hi! I’m Geoff Pynn.
我在北伊利诺伊大学教哲学
and I teach philosophy at Northern IIIinois University.
早些时候 在视频《批判性思维介绍》中
In my earlier Introduction to Critical Thinking video,
我描述了演绎论证和扩充论证之间的区别
I described the difference between deductive arguments and ampliative arguments.
在后续视频中
In the next few videos,
我将更多谈及每种论证的详情
I’ll talk a bit more about each type of argument.
咱们先从演绎论证开始吧
Let’s start with deductive arguments.
论证就是一组被称为前提的陈述
An argument is a set of statements, called its premises,
为其它陈述提供使人信服的理由
that are meant to give you a reason to believe some further statement
而其它陈述就是论证的结论
called the argument’s conclusion.
在一些论证中 只有前提为真
In some arguments, the premises are meant to guarantee
才能保证结论为真
that the conclusion is true.
这些论证被称为演绎论证
Arguments like this are called deductive arguments.
正确的演绎论证给你充足的理由相信其结论
A good deductive argument can give you a very good reason for believing its conclusion.
因为 它能保证结论为真
After all, it guarantees that its conclusion is true.
但并不是所有的演绎论证都是正确的
But not all deductive arguments are good,
所以当你判断某个演绎论证的结论是否为真时
and so there are several things to think about when deciding whether
应该考虑以下问题
to believe the conclusion of a deductive argument.
正确的演绎论证确实能保证其结论为真
A good deductive argument really does guarantee its conclusion.
也就是说
Part of what this means is that
不可能出现前提正确 而结论错误的情况
it’s impossible for the premises to be true while the conclusion is false.
在这种情况下 我们就说论证是有效的
When this is the case, we say that the argument is valid.
valid(有效的)是论证方法专用术语
Now this is a special, technical use of the word “valid.”
在日常生活中 我们用这个词来表示
In ordinary life, we often use this word to mean
好的、有说服力的、合理的等意思
something like good, cogent, or reasonable.
比如如果你不同意某人的观点
Like if you’re disagreeing with someone about something,
就此 他向你阐述了一些似乎很合理的建议
and they respond to a claim you make by saying something that seems pretty reasonable to you,
你会说:“哦 我认为你说的有道理”
you might say, “Well, I guess you have a valid point.”
这就是valid的常用意
Though that’s what the word often means in ordinary life,
但在论证方法上它的意思就变了
it’s not what the word means here.
当哲学家说一个论证有效时
When philosophers say that an argument is valid,
他们指的一直都是:
they always mean this very specific thing:
如果该论证的前提为真 那结论也一定为真
that if the premises are true, the conclusion must also be true.
Wi-Phi网站上还有一些视频
There are several other Wi-Phi videos that
里面更详细地讨论了“有效性”这个概念
discuss this notion of validity in more detail.
如果一个演绎论证是有效的
To say that an argument is valid is to say
那么该论证的前提和结论之间必然有联系
something about the relationship between the premises and the conclusion.
也就是说 如果前提为真
Namely, that if the premises are true,
结论也必然为真
the conclusion must also be true.
但并不是说 演绎论证的前提为真时 结论也为真
But it’s not to say that its premises or conclusion are true.
举一个例子来佐证这个观点
Consider, for example, this argument.
前提1:碧昂丝出生在法国
Premise 1: Beyonce was born in Paris.
前提2:出生在法国的人都爱吃奶酪
Premise 2: Everybody who was born in Paris loves cheese.
结论:因此碧昂丝爱吃奶酪
Conclusion: Therefore, Beyonce loves cheese.
此论证的前提是错误的
Those premises are false.
因为碧昂丝是在休斯顿出生的
Beyonce was born in Houston,
而且我打赌 肯定有在巴黎出生却讨厌奶酪的人
and I’m willing to bet that at least some people born in Paris hate cheese.
这时上述论证仍然是有效论证
Still, it’s a valid argument.
因为如果前提为真
If the premises were true,
结论必定为真
then the conclusion would have to be true.
但是如果前提为假
But because the premises are false,
那么该论证的结论就不是正确的
this argument doesn’t give you a good reason to believe its conclusion,
就算推理有效也于事无补
even though it’s valid.
哲学家称前提为真 推理有效的论证为可靠论证
Philosophers call a valid argument with true premises “sound”.
和单词“valid”(有效的)一样
Like the word “valid”,
这里的“sound”(可靠的)也不取其常用意
the word “sound” is term with various meanings in ordinary life,
该词常用于描述合理的令人信服的言论
and it can be used to describe some claim as reasonable or compelling.
但当哲学家说论证可靠时
But when philosophers describe an argument as sound,
他们指的从来都是:
they always mean this very specific thing:
该论证推理有效 且前提都为真
that it’s valid, and that its premises are in fact true.
再举个有关可靠论证的例子
Here’s a pretty boring sound argument.
前提1:碧昂丝出生在休斯顿
Premise 1: Beyonce was born in Houston.
前提2:出生在休斯顿的人都出生在德克萨斯州
Premise 2: Everybody who was born in Houston was born in Texas.
结论:因此碧昂丝出生在德克萨斯州
Conclusion: Therefore, Beyonce was born in Texas.
如果想了解“有效论证”这个概念的更多论述
For more discussion of the concept of a sound argument,
请观看亚伦·坎塞尔的Wi-Phi视频《可靠性》
see Aaron Ancell’s Wi-Phi video entitled “Soundness”.
所以 在判断演绎论证的结论是否为真时
So, before deciding whether to believe the conclusion of a deductive argument,
你需要判断该论证是否为可靠论证
you need to determine whether the argument is sound.
也就是说 你既需要判断推理的有效性
And this, in turn, requires determining whether the argument is valid,
又需要判断前提的真实性
and whether its premises are true.
那么 你怎么知道论证是否有效呢?
Well, how do you tell whether an argument is valid?
有时推理的有效性一目了然
Sometimes, it’s just obvious.
但通常情况下并非显而易见
But often, it’s not so obvious.
判断一种论证是否有效的一种方法是
One way to figure out whether an argument is valid
看你能不能举个反例
is to see if you can think of a counterexample to it.
这个反例论证可以是真实的 也可以是虚构的
A counterexample is a case, either real or imaginary,
只要它前提为真 结论为假即可
where the argument’s premises are true, but the conclusion is false.
所以请看下面这个论证
So, for example, consider this argument.
前提1:古典音乐家喜欢歌剧
Premise 1: Classical musicians appreciate opera.
前提2:碧昂丝是流行歌星 不是古典音乐家
Premise 2: Beyonce is a pop star, not a classical musician.
结论:所以碧昂丝不喜欢歌剧
Conclusion: Therefore, Beyonce doesn’t appreciate opera.
现在我们假设碧昂丝从小到大一直听歌剧
Now, suppose that Beyonce’s been listening to opera since she was a little girl,
并且非常喜欢莫扎特的《唐·乔望尼》
and loves Mozart’s Don Giovanni.
所以碧昂丝喜欢歌剧
Well, then she’d appreciate opera.
由此可见上述论证前提为真 结论为假
The conclusion would be false, even though the premises would still be true.
前提1“古典音乐家喜欢歌剧”为真
It would still be true that classical musicians appreciate opera,
前提2“碧昂丝是流行歌星 不是古典音乐家”为真
and that Beyonce is a pop star, not a classical musician.
但反例证明了该论证的推理无效
This counterexample shows that the argument isn’t valid,
所以就算前提为真
and so that even if premises are true,
但是推理无效 结论就不能为真
the argument doesn’t provide you with a reason to believe its conclusion.
还有其它更正式的方法帮你弄清推理是否有效
There are other, more formal techniques for figuring out whether an argument is valid,
我们会在接下来的视频中讨论那些方法
which we’ll hopefully be able to discuss in future videos.
所以如果你不确定演绎论证的前提是否为真
Now, if you don’t know whether the premises of an argument are true,
那么即使推理可靠
then even if the argument really is sound,
结论也不一定为真
it doesn’t give you a good reason to believe its conclusion.
如果你知道推理有效 但不确定前提是否都为真
When you know that an argument is valid, but you don’t know whether its premises are true,
那么推理充其量就是使结论为真的条件之一
the argument gives you, at best, a conditional reason to accept its conclusion.
但如果前提也为真
If you learn that its premises are true,
那结论必然为真
then you’ll have to accept its conclusion.
那么你如何判断论证的前提是否为真呢?
So, how do you tell whether an argument’s premises are true?
这时候逻辑和哲学也帮不了你多少
Well, this isn’t the kind of thing logic or philosophy can give you much help with.
为了弄清论证的前提是否为真
To figure out whether an argument’s premises are true,
你需要做一些调查
you need to do some research.
所以一个好的批判性思考者不仅需要逻辑能力
This is one reason why being a good critical thinker requires more than just logical ability.
还需要大量的常识 丰富的经验
It also takes a lot of real world, empirical knowledge.
除非你知道的足够多 能够确定论证的前提为真
Unless you know enough to know whether an argument’s premises are true,
不然就算你是优秀的逻辑学家 能进行有效推理
then even if you’re a really brilliant logician and know that the argument is valid,
你也不能保证结论的正确性
it doesn’t give you reason to believe its conclusion.
你知道的越多
The more you know,
对演绎论证的把握就越得心应手
the better you’ll be able to evaluate deductive arguments.

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视频概述

本视频为你介绍论证两大基本方法之一:演绎论证。

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视频来源

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

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