How Do You Know You Are Not Dreaming? 你怎麼知道現在不是在作夢？

🔊René Descartes made this question the starting point of modern philosophy. In the Meditations of 1641 he describes himself as a man who has noticed how many false opinions he took for granted in his youth, and who resolves, once in his life, to demolish everything and begin again from the foundations. His instrument is doubt, used as a test. He will set aside any belief for which he can find the slightest ground of doubt, in order to see whether anything is left. It is important to understand that he does not think his beliefs are false. He treats them the way one treats a basket of apples when some may be rotten: tip them all out, and put back only those that pass inspection.

笛卡兒把這個問題變成了近代哲學的起點。在 1641 年的《沉思集》裡，他描述自己是這樣一個人：他注意到自己年輕時把多少錯誤的意見視為理所當然，於是決心一生一次，把一切拆毀，從地基重新開始。他的工具是懷疑，拿來當作一種檢驗。任何信念，只要他找得到一絲懷疑的理由，就先擱在一邊，看看最後還剩下什麼。要緊的是明白：他並不認為自己的信念是假的。他對待它們的方式，就像對待一籃可能有幾顆爛掉的蘋果：全部倒出來，只把通過檢查的放回去。

🔊The doubt comes in three waves, each stronger than the last. First, the senses sometimes deceive: a square tower looks round from far away. But this touches only what is distant or small. It gives no reason to doubt, as he says, “that I am in this place, seated by the fire, clothed in a winter dressing gown.” The second wave reaches that as well. He has often dreamed of sitting by the fire while he lay undressed in bed, and on reflection he finds “that there exist no certain marks by which the state of waking can ever be distinguished from sleep.”

懷疑分三波而來，一波比一波強。第一波：感官有時會騙人，方塔從遠處看是圓的。但這只碰得到遠的或小的東西；它並沒有給人理由去懷疑，如他所說，「我在這個地方，坐在爐火旁，穿著冬天的睡袍」。第二波連這一點也淹過去了。他常常夢見自己坐在爐火旁，其實當時正脫了衣服躺在床上；細想之下他發現：「沒有任何確定的標記，可以把清醒的狀態跟睡眠區分開來。」

🔊Even so, something seems to hold out against the dream. Whether he is awake or asleep, “two and three make five, and … a square has but four sides.” For these he needs a third and more radical supposition: that “some malignant demon, who is at once exceedingly potent and deceitful, has employed all his artifice to deceive me,” so that he goes wrong even when he adds. Nobody believes in the demon, and Descartes did not either. Its job is to mark the outer limit of what can coherently be doubted.

即便如此，似乎還有東西抵擋得住夢。不管他是醒是睡，「二加三等於五，……正方形只有四條邊」。要動搖這些，他需要第三個、更徹底的假設：有「某個邪惡的魔鬼，既極其有力又善於欺騙，用盡了他一切的手段來騙我」，以致於他連做加法都會出錯。沒有人相信這個魔鬼存在，笛卡兒自己也不信。它的任務是標出「可以前後一致地懷疑」的最外緣。

🔊Nearly two thousand years earlier, the Chinese philosopher Zhuangzi had told the most famous dream in literature. He dreamed he was a butterfly, fluttering about, content, with no idea that he was Zhuang Zhou. Suddenly he woke, and there he was, solidly Zhuang Zhou. “He did not know whether he was Zhou who had dreamed he was a butterfly, or a butterfly dreaming he was Zhou.”

比這早將近兩千年，中國哲學家莊子講了文學史上最有名的一場夢。他夢見自己是一隻蝴蝶，翩翩飛舞，自在得意，完全不知道自己是莊周。忽然醒來，他又明明白白是莊周了。「不知周之夢為胡蝶與，胡蝶之夢為周與？」

🔊The premise is Descartes’ and the mood is entirely different. Descartes is alarmed and wants certainty back. Zhuangzi is unworried. He adds that between Zhou and the butterfly there must be some distinction, and calls this “the transformation of things.” Elsewhere in the same chapter he suggests that only after a great awakening will we know that all this was a great dream, while fools believe that they are already awake. For him the dream is a way of loosening our confidence in fixed distinctions: self and other, waking and sleeping, perhaps life and death.

前提跟笛卡兒的一樣，心情卻完全不同。笛卡兒是驚慌的，他要把確定性找回來。莊子並不擔心。他接著說，莊周與蝴蝶之間必定有分別，並把這稱為「物化」。在同一篇的另一處他提到：要有一場大覺，然後才知道這一切是一場大夢，而愚昧的人自以為已經醒了。對莊子來說，夢是一種方法，用來鬆開我們對固定區分的信心：我與物、醒與睡，也許還有生與死。

🔊At the bottom of his doubt Descartes finds one thing the demon cannot take away. Let the demon deceive him as much as it can: to be deceived he must exist. “This proposition I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind.” The point is usually called the cogito, from the Latin for “I think.” Notice how little it gives him. It tells him that he exists for as long as he is thinking, and that he is at least a thing that thinks. It does not yet tell him that he has a body, that he has a past, or that anything exists outside his mind. Descartes spent the remaining four Meditations trying to win the world back, by way of a proof that God exists and is no deceiver. Few readers have been persuaded by that route. The doubt has proved more durable than the cure.

在懷疑的最底層，笛卡兒找到一樣魔鬼拿不走的東西。任憑魔鬼怎麼騙他：要被騙，他就必須存在。「『我是，我存在』這個命題，每當我說出它、或在心裡想到它的時候，必然是真的。」這一點通常稱為 cogito，取自拉丁文的「我思」。注意它給他的有多麼少。它告訴他：只要他在思考，他就存在；而且他至少是一個會思考的東西。它還沒有告訴他：他有身體、他有過去，或者他的心靈之外有任何東西存在。笛卡兒用剩下的四篇沉思，試著把世界贏回來，走的是「證明上帝存在、而且不會騙人」這條路。很少讀者被這條路說服。懷疑比解藥耐久得多。

🔊The position that we cannot know anything about the external world is called skepticism, and three kinds of reply are worth knowing. The first is G. E. Moore’s. In a lecture of 1939 he held up his hands and said, in effect: here is one hand, and here is another; so there are external things. He granted that he could not prove he was not dreaming. His point was that he was far more certain of his hands than he could ever be of the premises of any philosophical argument to the contrary, so it was the argument that had to go.

主張我們對外在世界一無所知的立場，稱為懷疑論；有三類回應值得知道。第一類是摩爾的。在 1939 年的一場演講裡，他舉起雙手，大意是說：這裡有一隻手，這裡還有一隻；所以外在的東西是存在的。他承認自己無法證明不是在作夢。他的重點是：他對自己雙手的確定程度，遠遠超過他對任何相反的哲學論證之前提所能有的確定程度；所以該被放棄的是那個論證。

🔊The second reply denies that knowledge requires certainty. On this view, called fallibilism, I know that I am awake if my belief is true and is produced in a reliable way, even though I cannot rule out every imaginable alternative. For all I know in the skeptic’s sense, I might be dreaming; in the ordinary sense, I know perfectly well that I am not. The third reply questions whether the skeptic’s story makes sense. Hilary Putnam argued in 1981 that a brain which had always been kept in a vat and fed its experiences by a computer could not even think the thought “I am a brain in a vat,” because its words would never have been connected to real brains and vats. None of these replies is accepted by everyone, which is why the dream is still discussed in every introductory course.

第二類回應否認知識需要確定性。照這個稱為「可錯論」的看法，只要我的信念為真，而且是以可靠的方式產生的，我就「知道」我醒著，儘管我無法排除每一個想像得到的其他可能。照懷疑論者的意思，就我所知，我有可能在作夢；照日常的意思，我清清楚楚知道我沒有。第三類回應質疑懷疑論者的故事到底說不說得通。普特南在 1981 年論證：一顆從來就被養在桶子裡、由電腦餵給它種種經驗的大腦，連「我是一顆桶中之腦」這個想法都想不出來，因為它的字詞從來沒有跟真正的腦與桶子連結過。這些回應沒有一個是人人接受的；所以每一門哲學導論，至今還在討論那場夢。

🔊It is tempting to say that none of this could make any practical difference, and in one sense that is true. You will not tell dreaming and waking apart any better after reading Descartes. What changes is your picture of what knowing is. Before the dream argument it is natural to think of knowledge as a direct grasp of how things are. After it, one has to choose. Either knowledge demands a certainty that almost nothing can meet, or it is something more modest: true belief that is well supported and could, in principle, have been wrong. Most philosophers today take the second view. So do the sciences, which hold even their best theories open to revision. The dream argument is where that modesty was first learned.

人很容易說：這一切不可能造成任何實際的差別。在某個意義上這是真的。讀完笛卡兒，你分辨夢與醒的能力並不會變好。改變的是你對「知道」是怎麼回事的圖像。在夢的論證之前，人很自然地把知識想成對事物實況的直接掌握。在那之後，你必須做選擇：要嘛知識要求一種幾乎沒有東西達得到的確定性；要嘛知識是比較謙遜的東西——有良好根據的真信念，而且原則上本來有可能是錯的。今天多數的哲學家採取第二種看法。科學也是如此：即使是最好的理論，科學也保留修正的餘地。這份謙遜，最早就是從夢的論證學來的。