大學時代與一名同窗好友討論過這個問題：「何謂科學？」 ── 最後得到一個雙方都感到滿意的答案：「科學就是一堆可以被証明是對或者是錯的命題，而証明可以是基於邏輯，亦可以是透過經歷。」﹝也許前者就是理論，後者就是實驗吧！﹞
十多年來我都沒有質疑過這個定義的真確性，因為我根本沒有認真地思考。如今一想，上述「定義」的確是個非對即錯的命題；問題是：這個命題可以被証明嗎？如果可以，我又想不出方法來；若然不可以，那我只能慨嘆一句：「科學」真喺唔科學！
兩星期前簡略地讀過 Karl Popper 的名著 The Logic of Scientific Discovery 。何謂科學？ Popper 是這樣說的：
"[N]o matter how many instances of white swans we may have observed, this does not justify the conclusion that all swans are white."
"[E]xperience[...]can in the first place be only a singular statement and not a universal one. [...]Thus to ask whether there are natural laws known to be true appears to be only another way of asking whether inductive inferences are logically justified."
"[P]rinciple of induction must be a synthetic statement[...It] must be a universal statement. [...]To justify it, we should have to employ inductive inferences; and to justify these we should have to assume an inductive principle of a higher order[...T]he attempt to base the principle of induction on experience breaks down, since it must lead to an infinite regress."
"[T]here is no such thing as a logical method of having new ideas[...E]very discovery contains 'an irrational element', or 'a creative intuition'[...]"
"[I]nference to theories, from singular statement which are verified by 'experience'[...]is logically inadmissible. Theories are, therefore, never empirically verifiable[...N]ot the verifiability but the falsifiability of a system is to be taken as a criterion of demarcation."
"Science is not a system of certain, or well-established, statements; nor is it a system which steadily advances towards a state of finality[...] We do not know: we can only guess. And our guesses are guided by the unscientific, the metaphysical[...]faith in laws, in regularities which we can uncover - discover."
在這位哲學大師眼中，科學並非限於邏輯，因為世人只可憑經歷得出單稱命題﹝ singular statement ﹞，而自然定律皆是全稱命題﹝ universal statement ﹞。全稱命題只可從單稱命題憑邏輯歸納出來，而歸納原理﹝ Principle of Induction ﹞本身須是全稱的，否則歸納出來的命題就不可能是全稱的；但既然世人的經歷都是單稱的，全稱的歸納原理亦只可能是個無從證實的假說。換言之，世上存在自然定律，即所謂「萬物皆有序」只是信念﹝ belief ﹞。
科學講求實證，但既然經歷是單稱的，全稱的自然定律就不可能以經歷來確定﹝ verify ﹞ ── 縱使見過的一千隻天鵝都是白色，豈知第一千零一隻不會是白色而是黑色？只要世上有一隻黑天鵝，「世上只有白天鵝」這「定律」便遭否定﹝ falsify ﹞。因此，科學應以否證﹝ falsification ﹞為依歸。無論如何，以經歷為鑑，就是相信因果關係﹝ causality ﹞ ── 「事出必有因」：事件為果，必存在某自然定律為其因。
「萬物皆有序」與「事出必有因」都只能信，不可能不信。若說自然定律不存在，在科學的框架中，我們便應嘗試否定這個假說，那正正是在尋找自然定律，自打嘴巴。因果關係就更玄妙，你懷疑，做實驗意圖否定它，得出的是因還是果呢？除了疑惑，你還可以怎樣？
所謂科學，骨子裡只是一份信念 ── 深信「萬物皆有序」、篤定「事出必有因」；基於這份信念作出假設，並嘗試以實證方法于以否定……。
說到底，科學是不容質疑的信仰。
