林語堂老先生在他寫的蘇東坡的傳記《The Gay Genius: Life and Times of Su Tungp》中有一段特別好的話，剛好可以用來回答這個問題。
林語堂老先生的原文是用英文寫的，這裡引用一下張振玉的譯本《蘇東坡傳》對那段文字的翻譯，英文原版我附在後面：
中國有一句諺語，就是說一個人如何要“蓋棺論定”。
人生如夢，這出戏演得如何，只有在幕落之時才可以下斷語。不過有這種區別——人生是如同戲劇，但是在人生的戲劇裡，最富有智慧與最精明的伶人，對於下一幕的大事如何，也是茫然無知。但是真正的人生，其中總包含有一種無可避免的性質，只有最好的戲劇才庶乎近之。
因此在給過去的人寫一本傳記時，我們能把一場一場已經完成的戲逐一觀看，觀看由人內在的氣質與外在的環境所引起的必要的發展，這自然是一項重大的方便。
在我將《蘇東坡傳》各章的資料鑽研完畢之後，並且瞭解了為什麼他非要有某些作為不可，為什麼非要違揹他棄官歸隱的本意，我覺得自己好像一箇中國的星象家，給一個人細批終生，預卜未來，那麼清楚，那麼明確，事故是那麼在命難逃。中國的星象家能把一個人的一生逐年斷開，細批流年，把一生每年的推算寫在一個摺子上，當然卦金要遠高出通常的卜卦。
但是傳記家的馬後課卻總比星象家的馬前課可靠。
There is a current Chinese saying that final judgment upon a man is possible only when the cover is nailed on his coffin.
A man's life is like a drama, and we can judge a drama only when the curtain drops. There is this difference —— a man's life is a drama in which the wisest and shrewdest actor does not know what comes in the next act. But real human life always evolves with an inevitability which only the best drama approaches.
There is, therefore, a great advantage in writing the biography of a man of the past, where we can review scene after scene already completed, watching the inevitable development of events arising out of the necessity of outward events and inner temperament.
After I had completed research on the chapters of Su Tungpo's life and understood why he had to do what he did, against his deep and sincere urge to forsake politics and retire, I felt as if I were reading the predictions of a man's entire life by a Chinese astrologist, clear definite, inescapable. Chinese astrologists are able to plot the course of a person's entire life year by year, and are willing to put the whole prediction down in writing for a substantially higher sum than usual.
But the hindsight of biographers is always better than the foresight of astrologers.
感謝林語堂和張振玉兩位先生。
