"Paradigm shift" 這個詞組其實源自一本書 ── The Structure of Scientific Revolutions ﹝《科學革命的結構》﹞，作者是 Thomas Kuhn ，他原本是物理學家，後來醉心研究科學史，從中有所領悟，並於 1962 年寫成該書，提出甚具哲學意義、且顛覆性的見解。
現在在書店買得到的英文版多是五十周年紀念版，比起之前的第三版，該版加了 Ian Hacking 所著的引言。只因我喜歡原汁原味，不含雜質，為買 1996 年出版的第三版逛了多間書店，幸好在銅鑼灣的誠品找到一本。
據 Thomas Kuhn 所言，科學發展不是知識的持續累積，而是一個接一個的典範﹝paradigm﹞，從一個典範過渡至另一個典範的過程謂之「典範轉移」﹝paradigm shift﹞，促成典範轉移的是「危機」﹝crisis﹞以及隨之而來的「科學革命」﹝scientific revolution﹞。
典範是「常態科學」﹝normal science﹞發展的歷程，在一個典範之中，科學家普遍認為某些認知是真理，他們以這些認知為基礎進一步以「解答疑問」﹝puzzle-solving﹞的形式探索大自然及解釋自然現象，知識持續累積，科學得以平穩發展。在潛移默化中，他們做實驗的方法及所用的儀器、量度的標準，以至解釋實驗結果的理論都是基於這些認知制定出來，而課本及學校所教授的亦是這些認知，沒有人會懷疑這些認知的真確性；縱然某些發現與這些認知有抵觸，世人亦只會抱懷疑態度來看待這些發現，甚至將其視為「異象」﹝anomalies﹞。
當越來越多「異象」出現令越來越多科學家懷疑他們原有的認知，典範便陷入「危機」；可是部分科學家仍對他們的認知充滿信心，並認定遲早可以以現有的認知，藉「常態科學」的發展找出解釋「異象」的方案。
有科學家成功以新的認知解釋「異象」，新的認知獲得越來越多科學家的注意及認同，掀起一場「科學革命」，而隨著時光流逝，仍堅持舊認知的「死硬派」相繼逝世，舊的認知遭摒棄、遺忘、改寫，典範轉移完成。在新的典範之中，科學家普遍認為新的認知才是真理，在新的認知之下，他們對舊的觀測結果、概念、定義甚至是科學詞彙都有了新的理解，課本亦遭改寫。新一代的科學家毫不懷疑地認為這些認知是真理，並以此為基礎進一步探索大自然，新的知識又在「常態科學」的步伐下持續累積，科學再次得以平穩發展 …… ，直至另一個典範轉移發生。
在書中，Thomas Kuhn 多次以 Gestalt Switch 來解釋 paradigm shift；但又教人注意兩者其實不盡相同，例如：
[...A] process that involves "handling the same bundle of data as before, but placing them in a new system of relations with one another by giving them a different framework." Others who have noted this aspect of scientific advance have emphasized its similarity to a change in visual gestalt: the marks on paper that were first seen as a bird are now seen as an antelope, or vice versa. That parallel can be misleading. Scientists do not see something as something else; instead, they simply see it. [...T]he scientist does not preserve the gestalt subject's freedom to switch back and forth between ways of seeing. Nevertheless, the switch of gestalt, particularly because it is today so familiar, is a useful elementary prototype for what occurs in full-scale paradigm shift. (p.85)
The subject of a gestalt demonstration knows that his perception has shifted because he can make it shift back and forth repeatedly while he holds the same book or piece of paper in his hands. Aware that nothing in his environment has changed, he directs his attention increasingly not to the figure (duck or rabbit) but to the lines on the paper he is looking at. Ultimately he may even learn to see those lines without seeing either of the figures, and he may then say[...]that it is these lines that he really sees but that he sees them alternately as a duck and as a rabbit. [...]Unless there were an external standard with respect to which a switch of vision could be demonstrated, no conclusion about alternate perceptual possibilities could be drawn. [//]With scientific observation, however, the situation is exactly reversed. The scientist can have no recourse above or beyond what he sees with his eyes and instruments. If there were some higher authority by recourse to which his vision might be shown to have shifted, then that authority would itself become a source of problems[...]. The same sorts of problems would arise if the scientist could switch back and forth like the subject of the gestalt experiments. [...]In the sciences, therefore, if perceptual switches accompany paradigm changes, we may not expect scientists to attest to these changes directly. [...]Rather we must look for indirect and behavioral evidence that the scientist with a new paradigm sees differently from the way he had seen before. (p.114-115)全書連一幅插圖也沒有，為了解 Gestalt switch，在亙聯網用搜尋器搜了一番，雖然找不到以鳥及羚羊﹝bird and antelope﹞為題材的錯覺畫，但就找到「鴨與兔」﹝如下﹞，看過必能體驗 Gestalt switch。有說「鴨與兔」是心理學家 Joseph Jastrow 於 1888 年提出的：
Thomas Kuhn 的學說有以下的深層次含意，亦是備受爭議之處：
整體論﹝holism﹞
不同的典範有不同的架構，就連表面上客觀的觀測結果都會在不同的理論及觀念之下而有不同的理解﹝theory-laden data﹞，典範之間根本不存在共通的語言，故不可能共融﹝incommensurate﹞，新典範的出現宣告舊典範的覆亡。
相對主義﹝relativism﹞
在不同的典範之中，科學家對「真理」有不同的理解。至於那個「真理」才是真理？我們卻無從得知。絕對的真理並不存在；真理是相對的。
科學的非理性﹝irrationality﹞
世上不存在選擇典範的規則系統，轉移與否以及轉移至哪一個典範取決於科學界對不同典範「解答疑問」﹝puzzle-solving﹞的潛力的評估。此外，由於絕對的真理不存在，探索真理亦難說得上是科學發展的目標，科學的發展正如達爾文在《進化論》提出的生物進化過程，生物隨自然環境的轉變而進化，科學則在不同的典範中向不同的「真理」邁進，沒有永恆的目標。
參考資料：
- The Structure of Scientific Revolutions, Third Edition, Thomas S. Kuhn, The University of Chicago Press, ISBN-10: 0-226-45808-3
- Philosophy of Science: A Very Short Introduction, Samir Okasha, Oxford, ISBN: 978-0-19-280283-5
