一个由剑桥大学外语考试部热推引发的扯淡

高老师叨叨前几天刷微博的时候，看到剑桥大学外语考试部提问，what is one of the shorte...

高老师叨叨

前几天刷微博的时候，看到剑桥大学外语考试部提问，what is one of the shortest complete sentences in the English language. 答案是I am. 这条让我想到高中时期曾跟剑桥大学哲学系教授Ed Piercy学习西方哲学，死磕知识论（epistemology）近一年。今天跟大家聊聊知识论中的A priori，a posteriori和笛卡尔。

说

A priori

A priori是一个拉丁文，指的是独立于经验的知识或者论证(knowledge or justification is independent of experience)。比如说如果我们有基本的数学概念，那么我们可以证明7+5=12而不用诉诸于经验。

A posteriori也是拉丁文，与a priori相反，指的是基于经验或者经验性证据的知识或者论证（knowledge or justification is dependent on experience or empirical evidence）。一个经典问题是，how to discover whether a philosopher’s front door’s color is blue or white? 如何找到一个哲学家正门的颜色是蓝色还是白色？

理性主义和经验主义经常用a prioir和a posteriori探讨知识是如何被获得的。笛卡尔被认为是理性主义学派的代表人物之一，他认为reason has precedence over other ways of acquiring knowledge, 也就是说，通过理性思考获取知识的方式优于其他方式（如，经验主义的方式）。他在其名著Meditations on First Philosophy（第一哲学沉思集）对这个观点的论证方式非常精彩。

说

Doubt

首先，笛卡尔在Meditations卷I里用三个层次的疑问（waves of doubt）去怀疑是否任何知识是真实的（what can be called into doubt）。把所有基于错误基础上形成的知识都排除后，通过找到一个绝对正确的基础从新构建自己的知识体系。

第一个层次的怀疑：基于感官的错误，既perceptual illusion

人们通过感官所获取的知识（sensory knowledge）往往是具有欺骗性的。我们远眺长长的火车车轨，我们的视觉告诉我们车轨越来越窄，而实际上当然并非如此。因此，通过感官建立起来的知识并非牢靠。

第二个层次的怀疑：基于真实与虚幻的错误，既the dream problem

笛卡尔在书中说道，他爱睡觉（I am in the habit of sleeping）。他经常梦见自己穿着衣服，坐在火炉边思考，而实际上躺在床上并未穿着衣服。有多少梦境，我们根本无法分辨自己是处于梦中呢？（there exist no certain marks by which the state of waking can ever be distinguished from sleep）快速讲一下by which的用法，这句话正常语序是“the state of waking can ever be distinguished from sleep “by”no certain marks that exist. 这里which指代的是there exist no certain marks，为了强调“no certain marks”，把by提到了句子的前面。

笛卡尔接着提到，我们在梦境中所梦到的事物，即便是我们的想象，但这些想象总是代表着实际事物。笛卡尔用画师作画举了一个例子。这个句子用的句型结构非常变态，我给大家分析一下整体结构。

整句1：Painters themselves, even when they study to represent sirens and satyrs by forms the most fantastic and extraordinary,

平行1：cannot bestow （动词1）upon them natures absolutely new, but （逗号做平行，but转折）

平行2：can only make （动词2）a certain medley of the members of different animals; or （用冒号隔开两个整句）

整句2：if （用if做从句表假设，修饰后面主句it is certain）they chance to imagine something so novel that （从句that修饰something）

平行1：nothing at all similar has ever been seen before, and

平行2：such as is, therefore, purely fictitious and absolutely false,

整句2的主句：it is at least certain that the colors of which this is composed are real.

第三个层次的怀疑：基于全能上帝的怀疑，既a deceiving God

会不会有一个全能的上帝（an omnipotent God）吃饱了撑着没事儿干，使得笛卡尔所知、所认同都是错误得呢？其实天不是天，地不是地，1+2不等于3，而只是幻化出来的，欺骗他的假象？How do I know that he has not arranged that there should be neither earth, nor sky, nor any extended thing, nor figure, nor magnitude, nor place, providing at the same time, however, for the persuasion that these do not exist otherwise than as I perceive them?

但关键在于，我们无法判断，何时这个神淘气的在欺骗我们，或是没有欺骗我们。If, however, it were （这个地方为什么是it were而不是it was呢？因为是虚拟语气，既“假如”说神淘气得让笛卡尔不断受到欺骗）repugnant to the goodness of Deity to have created me subject to constant deception, it would seem likewise to be contrary to his goodness to allow me to be occasionally deceived; and yet it is clear that this is permitted.

笛卡尔宣布，基于第三个层次得怀疑，他现在放弃所有曾经视为真实、正确得事实。I will suppose that the sky, the air, the earth, colors, figures, sounds, and all external things, are nothing better than the illusions of dreams; I will consider myself as without hands, eyes, flesh, blood, or any of the senses, and as falsely believing that I am possessed of these. 那还有什么知识是真实的呢？

笛卡尔的结论：既然“我”被欺骗，那我本身一定是存在的因此才能被骗。I Am, I Exist。

Doubtless, then, I exist, since I am deceived; and, let him deceive me as he may, he can never bring it about that I am nothing, so long as I shall be conscious that I am something. So that it must, in fine, be maintained, all things being maturely and carefully considered, that this proposition I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind.

但笛卡尔所证明的“存在（exist）”是精神层面的存在，上面解释的三个层次的怀疑说明笛卡尔不信任感官经验（sensory information）和物质世界存在的本身（existence of a material world）。那笛卡尔相信什么呢？他相信他是一个思考的东西。。。What I really am is a mind or soul. I am a thing that thinks.

Language，Truth，and Logic

A.J.艾耶尔，英国哲学家，逻辑实证主义学派代表人物，很是不屑上面那套虚无缥缈的东西。他在其名著Language，Truth，and Logic（语言，真实和逻辑）中提出了实证原则，verification principle，既一个陈述是有意义的，如果听到这个陈述的人知道如何证实这个陈述，否则这个陈述要么是of literal insignificance（无意义），要么是tautology（同义反复）。A sentence is factually significant to any given person, if he knows how to verify the proposition which it purports to express, i.e., if he knows what observation would lead him to accept the proposition as being true or reject it as being false. In other words, a proposition is genuine when it is verifiable so that it is possible for experience to render proposition probable.

也就是说，A.J.艾耶尔只认可A posteriori knowledge。直到1959年苏联探测器Luna 3 space probe给月球的背面照了一张相片，在那之前从未有人看到过the far side of the moon（地球人啊，在地球上只能看到the near side）。但如果1959年之前有人宣称月球的far side有人，我们应该如何评价这句话呢？按照艾耶尔的观点，虽然我们彼时无法判断这句陈述的真假，但是理论上能被判断，那么这句陈述是有意义的。

但我们前面提到，A priori的真实性亦不容怀疑，笛卡尔说“我思考说明我是一个思考的东西”虽不涉及实证检验，但也没毛病。艾耶尔牛逼了，他说这是tautology，同义反复，ignore。

Ayer claims from a set of tautologies, taken by them, only further tautologies can be validly deduced. It is absurd that a system of tautologies constitutes the whole truth about the universe. So, you cannot expect to deduce any factual knowledge from general principle or so called first principle. A proposition like “A is A” cannot deduce any fact of any matter in our world.