证据是什么？
陈述观点就是告诉其他人你关于某事的想法；展示证据就是告诉其他人我们的想法是有意义的。表达比被告知更加有趣和印象深刻——我们在学校就知道这些了。那么，为什么，还有那么多文章和演讲由一个又一个观点堆积，但是却几乎没有证据支持它们。就像我们在第五章看到的，一个原因是人类的大脑是一个名副其实的观点工厂，所以绝大多数人都有大量的意见分享。另一个原因是人们倾向于只记得观点，而忘记了获得它们的经过，就好像学生在忘记了他们的考试和作业分数后很长时间还记得他们最后课程的等级。
在某种更意味深长的角度看，另一个原因是有时候很少有或者根本没有证据可以想起来——换句话说，观点根本没有任何实质性基础。例如，在1999年早期人们认为（William Jefferson Clinton’）克林顿在法庭宣誓后撒谎没有“上升到被弹劾的程度”。当被问道为什么这么想的时候，有些人用同样的或者类似的话重复他们的论断：“他不应该因为这些事情被赶出白宫”或者“这是他和希拉里的事情”。还有些人提供相关联的观点：“这是右翼的阴谋”或者“独立的检察官肯尼斯斯塔尔（Kenneth Starr）在进行政治迫害”。虽然无法确定他们为什么这么想这么做，他们用几乎一样的措辞表达观点，而这些观点是半打白宫顾问和无数其他克林顿的支持者不断重复的观点。这个事实表明了这些人只是简单地不经评估地接受了别人的观点。
我们都能认同这些人。我们绝大多数不想承认，当我们要找到理由支持观点时，我们会制造出最站不住脚的证据。我们可能会安慰自己说我们脑袋里有厚厚一叠证明文件没有整理好。但是非常可能的事情是这个毫无价值的证据就是我们的所有。批判性思考者被诱惑犯下同样的自我欺骗的错误，这些错误给其他人带来了麻烦，但是他们学到了抵制这些诱惑的价值。更重要的是，他们养成了在形成观点前核实证据的质量和数量的习惯。并且，他们在表达观点前重新复核证据。这样花费时间所获得的补偿远大于因为知道自己在说什么的信心。
证据的种类
评估你自己和其他人的观点，你就需要理解不同种类的证据。这需要知道不同种类证据的价值和局限性，还有如何问合适的问题。最重要的证据有个人经验、未发表的报告、已发表的报告、目击证人证言、名人证言、专家意见、实验、统计、调查、正式观察和研究综述。
特别需要注意的是，这里的安排不是按可靠性的升序或降序排列，而是按熟悉程度的大致顺序排列——个人经验对大多数人来说非常熟悉，研究综述则不太熟悉。
原文：
What Is Evidence?
To state an opinion is to tell others what we think about something; to present evidence is to show others that what we think makes sense. Being shown is much more interesting and impressive than being told—we’ve all known this since grade school. Why, then, does so much writing and speaking consist of piling one opinion on another, with little or no evidence offered in support of any of them? As we saw in Chapter 5, one reason is that the human mind is a veritable opinion factory, so most people have an abundance of opinions to share. Another reason is that people tend to remember their opinions and forget the process by which they got them, much as students remember their final grade in a course long after they have forgotten the tests and homework grades that resulted in it.
Another, and in some ways more significant, reason is that sometimes there is little or no evidence to remember—in other words, the opinion is based on nothing substantial. For example, in early 1999 many people held the opinion that William Jefferson Clinton’s lying under oath did not “rise to the level of an impeachable offense.” When asked to explain why they thought that, some people repeated the assertion in identical or similar words: “He shouldn’t be removed from office for what he did” or “It’s between him and Hillary.” Some offered related opinions: “It’s a right-wing conspiracy” or “Independent counsel Kenneth Starr is on a witch-hunt.” Though it is impossible to be certain why they thought as they did, the fact that they expressed the opinion in the very same words incessantly repeated by a half dozen White House advisors and innumerable other Clinton supporters suggests that they simply borrowed the opinion without evaluating it.*
We can all identify with those people. More often than most of us would care to admit, when called on to support our opinions, we manage to produce only the flimsiest of evidence. We may soothe ourselves with the notion that a thick folder of evidence lies misfiled in our minds, but the very real possibility remains that flimsy evidence was all we ever had. Critical thinkers are tempted to commit the same self-deception that plagues others, but they have learned the value of resisting that temptation. More important, they have developed the habit of checking the quality and quantity of the evidence before forming an opinion. Also, they review their evidence before expressing an opinion. The extra time this takes is more than compensated for by the confidence that comes from knowing what they are talking about.
Kinds of Evidence
To evaluate your own and other people’s opinions, you will need to understand the various kinds of evidence. This entails knowing the value and limitations of each kind, as well as the appropriate questions to ask. The most important kinds of evidence are personal experience, unpublished report, published report, eyewitness testimony, celebrity testimony, expert opinion, experiment, statistics, survey, formal observation, and research review.
It is important to note that the arrangement here is not in ascending or descending order of reliability but rather in rough order of familiarity— with personal experience being very familiar to most people and research review much less familiar.
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