儒家的經典《春秋》，原本是記載魯國的史事，經過孔子的筆削之後，才成為今日的規模。因為畏懼禮樂制度的日漸凌遲，所以仲尼將自己的褒貶意見，表現在字裏行間，目的是要讓諸侯國君有所戒懼。但由於文字太過簡略，當時受教的弟子，也無任何解釋經文的記錄流傳，導致漢代學者對同一《春秋》文本，產生不同的各家師說。至西漢末，《左傳》為劉歆重新發現，世人才開始對《左傳》的研究產生興趣。但《公羊》、《穀梁》既先出於前，且立學官，故多有引此二家之義，以解《左傳》事件，形成三傳有相互引用的情況。晉朝杜預，年少僻好《左傳》，為恢復其原本解經的面貌，他兼採各家的解釋，但拋棄公、穀之義。並在多年閱讀的經驗中，從文字中尋找規則，歸納出三種「義例」。最後將經與傳相附，使學者方便閱覽。用這種科學的歸納方法，解釋分析史書中的現象，杜預可說是超越前人。此篇文章，則是針對杜預〈集解序〉中所提及的方法，做進一步的說明，以期吾輩對《左傳》的義法，能有更深入的了解。

Confucian classic "Chuan Chio”, originally recorded in the history of the State of Lu things, after revision of Confucius, became today's scale. Because of the growing fear of lingering death ritual system, so Zhongni to appraise their own views, expressed in between the lines, the purpose is to make some fear toward those kings. However, the text is too brief, was taught the disciples, and no record of any interpretation of scripture spread, resulting in the Han Dynasty scholar of the same, " Chuan Chio " the division. To the late Western Han Dynasty, "Zuo Zhuan"as Liu Xin rediscovered, the world finally began on the "Zuo Zhuan" of interest. But “Gon Yang” Gu Liang, "both for the former first, and set school officials, so many have cited the two meaning, to alleviate the "Zuo Zhuan" incident, refer to each other to form athree-pass situation. Du Yu who had lived in Jin Dynasty, young hobby "Zuo Zhuan" solution to restore its original appearance by his adopting both the interpretation of each, but purports of Gon and Gu are excluded. And in the many years of experience in reading from the text to find the rules, identifies the three "Justice Cases. " Finally, compared with the pass laws, so that scholars easy access. Summarized in this scientific method of analysis to explain the phenomenon in the history books, Du Yu can be said to surpass their predecessors. This article, it is against the Du Yu "Variorum Preface"mentioned in the method for further instructions, to for us the right "Zuo Zhuan" of justice cases law, to have a better understanding.