The emergence and development of private international law can be said to be accompanied by the development of related theories, in which theory plays a very important role. When international disputes are treated differently from scratch, from generality to concreteness, jurists turn their attention to the field of international law. Since the earliest Roman period, the law of nations has become the bud of international law, and private international law has a relatively independent and mature system. Since its inception, the development of private international law has gone through four stages: embryonic stage, differentiation of rules, modern private international law and contemporary private international law. In the meantime, countless jurists worked hard for the reasonable and complete construction of private international law, but the focus fell on two aspects-the first is the reason why China applied foreign law when dealing with foreign-related civil and commercial disputes; The second is the method of applying foreign law. The first aspect is a basic theoretical basis in the application of private international law, because only on the basis of a reasonable application reason can we further discuss how to apply it. Therefore, scholars of private international law have made great efforts on this issue.

Just like the historical law of international disputes from scratch and accompanying transactions, this issue is raised and explored from scratch. This "nothingness" is understood as the reason why jurists have not demonstrated the application of foreign laws with proper reasons. There are two situations: for jurists who emphasize the territorial effect of law, there is no need to demonstrate reasons; Emphasizing the personal effect of law is to ignore this problem with the universality of law itself, that is, to determine the necessity of applying foreign law with the legal nature stipulated by people and to emphasize the necessity of applying foreign law with the legal obligations considered by jurists. For example, in the period of European racial law, it is obviously not desirable to emphasize that the law belongs to human nature and that all ethnic groups should apply their own laws to answer the reasons why foreigners apply laws in their own countries, because the application of foreign laws should be mutual among national subjects, and it is obviously unconvincing to regard natural persons as the subject of understanding under the framework of national judicial power. During this period, both personal effect and territorial effect were emphasized, and they failed to escape the shackles of the above two situations until the emergence of "international comity theory" broke this situation.

Huber's theory of international comity explains why a country applies foreign laws by respecting the laws of other countries through the mutual comity of rulers. It can be seen that this kind of application is due to the comity between sovereign countries, and this comity is mutually needed and has practical significance. This is the main point of international comity theory. Of course, this must be limited to national sovereignty and people's interests, so as to accelerate the development of international exchanges. I think the theory of international comity is the earliest reasonable explanation of applying foreign law in dealing with foreign-related disputes, and it is also convincing. It is feasible from the point of view of legal obedience and practical utility. With the deepening of theoretical research, the disadvantages of the theory of legal distinction are gradually revealed, so history has entered the development period of modern private international law, which emphasizes the analysis and research on the nature of legal relations. During this period, many famous schools have appeared to explain and solve the understanding of private international law, such as the theory of legal relationship, the theory of nationality law and the theory of acquired rights. And solve the application of foreign laws from different angles, including analyzing the reasons for applying foreign laws. Explaining the reasons for application through system theory is the solution to this problem by jurists in this period. For example, in the theory of location of legal relationship, the representative of savigny (1779- 186 1) searched for solutions to legal problems in foreign-related disputes by analyzing the location of the deepest connection of each legal relationship, that is, he used this location to answer why foreign laws should be applied-because for a legal relationship, its location is abroad.

The era has entered the development stage of contemporary private international law, and the boom of research on private international law has naturally produced many fruitful theoretical viewpoints, such as local law theory, government interest theory, preference theory, forum law theory, closest relationship theory and so on. Among them, the theory of closest connection is a very important principle in the choice of law when dealing with foreign-related issues, which shows that this theory has been accepted by most countries and become a universal theory. I also agree with this view, which is a profound and systematic theory about legal seats. At this point, I support Cook's "local law theory". Its main point is that the essence of the application of foreign laws is to "merge" foreign laws and regulations into domestic laws, so as to improve the consent of domestic laws, and the final result is to complete the end of the application of foreign laws, which is the embodiment of pure territorialism. I support this view because the research of legal theory needs to be forward-looking, which not only requires the theory to conform to people's wishes in practice, but also attaches importance to the direction of law in the future development and can promote this development with the power of theory. In the theory of "local law", it emphasizes the convergence of the application of foreign law and domestic law. The purpose of applying foreign law is to make creditor's rights based on the legislative norms of domestic law, in which foreign law is only for reference. When domestic laws and foreign laws are merged, the laws are unified. Countries realize the unification of national legal principles and even final rules through their own integration of private international law rules and the promotion of international conferences. This is the development trend within the framework of large international organizations. Therefore, I think the reasons for applying foreign laws should be viewed from two angles. From the perspective of legal application, through the analysis of legal relations, highlight the closest relationship, so as to correctly apply foreign laws; On the other hand, the purpose of applying foreign laws is to improve domestic legislation. The difference in principle and even the difference between domestic law and foreign law in specific rules should be developed satisfactorily in the process of reference and integration. When domestic law and foreign law are finally truly integrated, the answer to this question will come to a successful conclusion.

This paper is a class assignment of private international law.