西漢大儒董仲舒在〈三代改制質文〉－篇文字中提出了㆒個內容豐富、系統嚴密的古代歷史發展的理論。這個理論簡稱三統說，據學者考證是模倣戰國時期騶衍的五德終始說而建立的。騶衍的五德終始說在戰國後期即已十分流行，其影響所及包括秦朝及漢朝，秦始皇曾據此證明秦朝統治的正當性。漢初亦有士建議利用此說建立漢朝統治的正當性。董仲舒為何要捨棄騶衍之說而提出新的理論呢？本文之作旨在回答這個問題，探索可能的原由，其途徑是考察騶衍五德說的內在缺陷，以及漢初的政治、思想兩方面的情況。除此而外，本文最後試圖發掘董仲舒三統說對漢代法律文化的影響。

Tung Chung-shu, one of the great Confucian scholars in Han Dynasty, proposed in his article, “The Contents and Structure of Transforming Regime” (san-tai kai-chih chih-wen 三代改制質文), a theory of the development of history in the Three Dynasties, which was termed The Three Unities （三統）. Modern scholars have proved that the theory was modeled on the idea of Five Virtues Cycle（wu-teh chung-shihs 五德終始）, posited by Tsou Yan (鄒衍) of the Warring States period. Since Tsou’s idea was in vogue and circulated throughout the early stage of the Western Han Dynasty, why did Tung have to propose his new theory? This article attempts to look into the possible reasons by examining the nature of Tsou Yan’s idea, the political and intellectual milieu of the founding years of the Western Han Dynasty. The result proposed in this article might also help us to understand and evaluate the impact of Tung’s theory of Three Unities on the political culture of Han Dynasty.