晉初「《晉書》限斷」之議論，出現三種「起元」之說。此一現象顯示，論述朝代開端發生了歧見，將會影響帝王的正當性。本文析論其中「泰始為斷」的主張，何以成為此一論爭的最終抉擇。首先，探討三種「《晉書》限斷」的區別，以及此一議題何以在陸機擔任著作之職後，再次掀起議論。其次，討論「正始」、「嘉平」兩種起元模式的分別，與陸機否定「正始」、「嘉平」起年的理由。其三，從「泰始為斷」之說，討論晉武帝對於自身「受命」的建構；以及三國一統之後，歷史書寫對於「泰始」起元的重構，這將改變後世對於晉初的描述。

There are three theories explaining the beginning of the Jin dynasty, which shows that judgments of a dynasty’s beginning can influence the legitimacy of an emperor. This article analyzes the theory that considers the Taishi period as Jin’s beginning and explains why it has become the final choice among the three theories. The content of this article is divided into three parts. The first part investigates the three theories and the reason why this issue becomes popular again after Lu Ji assumes the role of official historian. The second part compares the theories that respectively takes the Zhengshi period and the Jiaping period as the beginning of the Jin dynasty and explains why Lu Ji is against them. The third part examines the Taishi theory to discuss how the Emperor Wu of Jin constructs his legitimacy and how historians who reconstruct the Taishi theory changes later descriptions of the beginning of the Jin dynasty.