正如哲學上存有諸多反覆的問題，每個哲學者不可避免會遭遇到，而在法學上，同樣的情形亦困擾著法學者。法學上最本原的問題，就是「法是什麼」的問題。對大多數法學者來說，對現行法的認識與應用，是學習法律的主要內容，至於「法是什麼」等較為抽象的思想問題，則因與現實司法的運作脫節，當然也就沒有深究答案的動力和必要。而且此類問題，卻往往難得唯一正解，就算放諸古往今來的法學大家，也未能給出令眾人心悅誠服的回答。 面對納粹的實證法不法，無論是法實證主義者或自然法論者都敗陣下來，本文從實證法不法問題的角度，來說明自然法與實證法思想根本的爭議─有無客觀正確之法。並在認識到自然法與實證法思想的難處之後，再借鑑柏拉圖的法思想，試著以不同於當代的視野，汲取柏拉圖思想的養份，看看能否解開這難解之結。 本文全部共分為五章，除第一章做為導論外，其餘四章做為主題論述，依其結構可分為三部分： 第一部分將簡單敘述自然法與實證法思想的歷史，並提出歷史上實證法不法的問題(特別是納粹德國時期的實踐後果)，說明後來法思想理論對立未決的部分，這些內容將是第二章論述的中心。 第二部分則進入柏拉圖的法思想討論，將先說明柏拉圖所處當時的希臘文化背景，以及其個人經歷，特別是蘇格拉底之死，然後再論述柏拉圖著作中所透露出來有關法律的思想，前面背景的部分會先在第三章予以處理，後面柏拉圖法思想與理論的論述部分則留到第四章討論。 第三部分為綜合論述，也是本文對所提問題的總結回應，將把古典的柏拉圖法思想，與現代的法思想理論結合起來，分析問題象徵的意義，思考能否得出對問題更好的回答，嘗試提出一些看法，此為第五章的內容。 / The perpetual problem of “what is Law” is always asked by the students, the professors, even the philosophers, but no real answer was or is made. The legal positivist like John Austin would declare “laws properly so called are commands of sovereign”, as to another hand, the supporters of theory of natural law would make a completely opposite claim. What shall we do or which side should we believe? In this essay, from the aspect of the contrast between “it ought to be” and “it is”, the history of theory of natural law and legal positivism is tracked down in order to try to figure out the problem of the unlawful laws which the Nazi had made and caused a terrible disaster. Because the result is not so content, so the thought of Plato is reseached as to find out if there would be something that is illuminating. The contents of all chapters of this essay could be digested as follows: In chapter one, as an introduction, it explains the reason that why the essay is written and how it is finished. In chapter two, it tracks down the history of theory of natural law and legal positivism, and shows a horrible instance that caused a catastrophe which neither legal positivism nor theory of natural law could undo. In chapter three, it refers the thought background of Plato including his life and the conditions of contemporary Greek that affected the behaviour and development of Plato. In chapter four, it discusses the thought of Plato such as the theories of form, justice, and legislation with further eyes on his ‘The Ruplic’, ‘Statesman’, and ‘The Laws’. In chapter five, some viewpoints are discussed in advance in order to conclude what is in question. And after that, a temporary conclusion is made.