本文試圖以多瑪斯的自然法為例，闡述西方近代法哲學所面臨的問題：合法性危機。由於西方近代思潮的趨勢是一種不斷世俗化的過程，最明顯的特徵，即是形上學的沒落、以及實證主義與無神論的興起。作為西方法哲學傳統的主要內涵的自然法，由於是以形上學與神學為其論證基礎，當然也隨之沒落。取而代之的則是以實證主義為其意識形態主導的實證法或成文法。而當代自然法的復興，當然是因為對於20世紀出現的人類歷史上從未經歷過的左、右兩派極權主義的反思。然而此一反思則必然要觸及自然法的合法性基礎。因此，由此衍生的一個不容迴避的問題則是：此一合法性基礎應該立基於形上學與神學，還是實證主義？本文一方面闡述了多瑪斯自然法的淵源：上帝信仰與希臘的理性主義傳統、尤其是Aristotle的思想;另一方面也闡述了其內涵，即著名的6個特點。本文並以當代著名法學家吳經熊與美國大法官Holmes關於「自然法應該是什麼」”此一問題的討論為例，說明其複雜性與矛盾。作為虔誠信徒的吳經熊，對於自然法的論證基礎當然是上帝所保證的普遍理性與真理：而對於生長於美國傳統中的Holmes，則當然是經驗。本文並嘗試指出，看似理性的美國自由主義，可能並非像它表面看起來那般理性，而其實是一種掩蓋了社會貧富不均的偽善的價值觀。以藉此闡明我們這個時代的一個基本困境：意識形態無所不在！

The author attempts to take the Thomistic Natural Law for example, to elucidate the problems which the modern Jurisprudence confronts, namely: the crisis of Legitimation. The trend of modern western thought is that of uninterrupted secularization. The most distinguished character for this, is the decline of metaphysics and the rise of positivism and atheism. Natural Law as the connotation of western legal philosophy has its ground in metaphysics and theology. Its decline is therefore consequential. That, which replaces it, is the positivistic ideology and the positive law. The renaissance of Natural Law in our times is due to the reflection on the unprecedented totalitarianism in the human history from both right and left wing in the 20. century. Such a reflection must touch the deep ground of the foundation of Legitimation for Natural Law. Therefore, it derives an inevitable question: Should such a foundation be built on metaphysics and theology, or rather on positivism? We discuss the origin of the Thomistic Natural Law: faith on God, and the Greek tradition of rationalism, especially the thought of Aristotle on the one hand, and its connotation, namely, the famous 6 characters on the other hand. We also take the discussion on “what the Natural Law should be” between WU Jingxung and Holmes as example, to shed light on its complex and contradiction. WU as a pious Christian justifies the Natural Law certainly on the ground of the universal ration and truth ensured by God, while Holmes as born in the American tradition takes the experience as its foundation. Moreover, we try to expose that, the rational seeming American Liberalism maybe not so liberal as it shines, but rather concealing the facts of social inequality with the hypocrite value, in order to elucidate one basic difficulty of our times: the Ideology is everywhere!