摘要： 本文以法经济学视角对法律体系的基本结构进行分析以揭示其背后的理性基础。人具有社会性又具有自私性，必须通过社会合作将二者加以协调。国家存在的合理性在于比单纯的社会自发调节在实现社会合作上成本更低。合作即意志行为的协调，包括“意志自由”型（自愿交易的合作）和“意志支配”型（强制的合作），它们都需要耗费一定成本。私法适宜前者，公法适宜后者。但是，市民社会生活中的许多交易并不是真正的“意志自由”，需要国家意志的强制干预来加以平衡；而国家干预会面临高昂的组织管理成本，需要适当引入自由协商机制。

关键词: 法律体系 , 私法 , 公法 , 社会法 , 交易成本

Abstract: This paper explores the fundamental divisions of the system of law in economic analysis to explain its rational base．It’S rational to constitute government for it costing lower than simply spontaneous regulation of civil society to promote social cooperate reconciling the human’S selfishness with his sociality．Cooperation is involved in the coordination between individuals，including free will coordination and constraint coordination，beth giving rise to certain costPrivate law appropriates the former，public law appropriates the latter．However’some transactions in civil society are not of one party’S own free will actually and need government intervenes to strike a balance．Government intervenes are usually confronted with high cost，and require introducing certain free bargaining mechanism．

Key words: system of law, private law, public law, social law, transaction cost