Introduction

Private international law is a separate branch of law that regulates private law relations with a foreign level, i.e. private law relations connected to several sovereignties. Condicio sine qua non for defining its subject is the existence of the foreign level in one of three elements of private law relationship (in the subject, object or in rights and obligations). The field regulating these relations is called the conflict of laws. Apart from the specific subject, this branch of law is also characteristic by the method of legal regulation that cannot be found in any other branch of law. Private international law resolves the legal problem arising with the emergence of the element of foreignness with the aid of the collision method. The structure of the collision rule significantly deviates from the classical structure of the legal rule, i.e. the disposition-sanction structure that is immanent to all other legal norms. The collision norm has two specific elements: the legal category and the connection point. The legal category may be most broadly defined as a legal issue or problem that gives rise to a dispute. The task of the collision norm is not to resolve the legal problem in a meritorious manner, but to use the connection point to refer to governing law (domestic or foreign) within which the contentious legal issue will be resolved. In that way, there is a possibility for the acting authority to deviate from the domestic law application and to apply foreign law as governing, if the connection point contained in the domestic collision norm refers it to that law and if the foreign law application, in this specific case, is not opposed to the institute of the domestic public order. The decision-making court, as a rule, seeks a solution relying on the collision norms of domestic law (lex fori).

A question arises whether private law relations are the ones in which the principle of every authority exercising only domestic law cannot be absolutely applied (Varadi et al., 2012, p.30). During several past decades there has been an increasing percentage of people and goods moving outside the boundaries of their own countries. Legal security would be threatened in its roots if these movements brought to question status, real, family, inheritance and other private rights of individuals, i.e. real and property rights to goods and in relation to goods that are the subject of international trade. Division of law into public and private originates from Roman law: "Ius publicum est quod ad statum rei publicae spectat, privatum autem quod ad singulorum utilitatem: sunt enim quaedam publice utilia, quaedam privatim, publicum ius in sacris, in sacerdotibus, in magistratibus consistit"1.

Economic and social conditions dictate what will be considered a private or a public sphere of law in a specific society at a specific time. The division of legal norms into public and private spheres is a dynamic category that does not necessarily mean the drawing of clear boundaries between them. This very drawing of boundaries, significant to the foreign law application, results from social and political conditions and circumstances. The public law sphere remains reserved for the strict application of the principle of territoriality that calls for every authority to apply only domestic law.

Beginnings of the foreign law application in ancient times

The slave society as the social order in ancient states denied the possibility of the foreign law application while even the domestic law exercise depended on the citizen's status. Greek states jealously guarded their civil honour and privileges for the so-called citizens by birth, although at a later stage they granted certain privileges to foreigners (metics) who could gain Greek citizenship. A special stratum of population was made of slaves who, as Aristotle states in his Politics, were nothing but "live tools", "active and different tools" available to their master (Encyclopaedia, 2008, p. 167). However, the existence of polises (city-states), their closeness and specific judicial system in Hellenic states gave rise to jurists who were familiar with the regulations of different city-states. The awareness and knowledge about the existence of diversity did not also imply the possibility of the foreign law application. In the set of these social circumstances, the element of foreignness could be found only in the possibility of an arbitrator from another polis being engaged to make an impartial decision in a dispute between the citizens of the original polis.

In ancient Rome there were two categories of free citizens: Roman citizens and other free people with the status of peregrines. The application of Roman law ius civile constituted a privilege of Roman citizens. A different legal system, or ius gentium, applied to peregrines – a system that was created through the action of praetors. When issuing edicts, a praetor sometimes happened to take into account customary or written law of one of the provinces. However, it was much more common for a praetor to create completely new and different rules. To that end, he could introduce some rules from local peregrine law in ius gentium (Bujuklić, 2018, p. 80). Strictly speaking, in such circumstances we cannot refer to the foreign law application as we see it today because the people who were subject to this law were bound to a single sovereignty. The praetor who created this law paid attention to the differences and specifics brought to the Roman state by the oppressed nations. Through ius gentium, Roman law attempted to bridge political, cultural and religious differences between Romans and the residents of the occupied provinces. This method is a predecessor of the idea of the harmonization and unification of legal rules as we know it nowadays. When the Edict of Caracalla from the beginning of the 3^rd century (212 AD) granted the rights of Roman citizens to peregrines, ius gentium became general law for all the nations living in the Roman state.

Romans were not familiar with the collision method, so it is one of the few concepts that were not created in Roman law, rightfully seen as the most magnificent legal system. Romans themselves had such a high opinion about their law that the following statement by Cicero comes as no surprise (Jezdić, 1960, p. 21): "Incredibile est qem sit omnia ius civile praetor, hoc nostrum incoditium as paene ridiculum".2

In the light of some theoreticians' thoughts about the European legal culture, Roman law has a two-fold role. The activity of Roman jurisprudents can be seen as the outcome of the "classical" European culture due to the intellectualism of Roman jurists. At the same time, Roman law is an open system demonstrating specific features that differ from the European legal tradition (Polojac, 2015, p. 177). It can also be concluded that the establishment of supranational law in the European Union had its predecessor dating centuries back to Roman law, within which ius gentium emerged and at a later stage got the attribute of general law applied to all citizens in the Roman state. Therefore it is reasonable to claim that the European Union law is not something completely new, but it constitutes a specific revival of a unique legal culture that used to cover whole Europe in the past (Stein, 2007, p. 144).

Medieval particularity

Social circumstances in the Middle Ages that cover the age of feudalism and the period of the establishment of absolutist monarchies were quite different. After the fall of the Western Roman Empire in 476 AD and the invasion of barbarians, there was no increasing probability of new conflicts. The reason for that is the broadest application of the principle of personality – the law of an ethnic community to which a person belonged was applied to all legal relationships of that person (Živković, 2004, p. 10). Sometimes the possibility of the foreign "law" application directly depended on the power of the community penetrating into a foreign territory. For example, the Vikings who were mostly Danes, while ravaging England in the 9^th century, settled in the greatest part of the English territory north of the Thames River. The area they settled in was known as Danelaw. In it, the English population also had to abide by Danish customs (Rutherfurd, 2009, p. 186).

Medieval particularity was reflected in different countries founded in the territory of the former Western Roman Empire, the consequence of which was different tribal affiliation that often gave rise to the question of what customs and rules should be applied when regulating a person's status. An interesting phenomenon is professiones iuris that emerged in the 8^th century and was quite widely used in the 9^th and 10^th centuries (Varadi et al., p. 232). Professiones iuris is a statement made by the party in a dispute as an answer to the question what law he/she lives in: Qua lege vivis? In literature, this statement is also seen as the first appearance of autonomy of the will in terms of the possibility given to the party to choose governing law.

With the strengthening power of feudal lords and feudal relations, the principle of territoriality suppresses the principle of personality and, as a rule, the same law is applied in the feud to all its residents. It was only with the Renaissance, the expanding trade relations and more liberal legal attitudes that the principle of territoriality was gradually abandoned (Pak, 1995, p. 219). In law, the Renaissance marked the recovery of the authority of Roman law, and in particular the authority of the Code of Justinian from 533 AD as the most important and influential legal codification. Law schools founded in the Renaissance period gave rise to the first doctrinarian debates dedicated to the conflict of laws, and created the collision method as a method specific to the problems of the governing law determination. In fact, during this period there was an interlocal conflict of laws that emerged in a relatively narrow area between the entities close to cultural, social and economic criteria. The very possibility of applying another entity's law was not brought to question in these circumstances.

The most significant contribution to the theory of private international law was made by three schools in the period from the 12^th to the 17^th century (Varadi et al., p. 233).

The work of the first one-the Italian statute school-is associated with two periods: the work of glossators (the 12^th and 13^th centuries) and the work of post-glossators (the 14^th and 15^th centuries). This school studied the conflict between customary rules in certain towns. The political circumstances at the beginning of 12^th century, created by the Peace of Constance from 1183, made fertile soil for the emergence of the conflict of laws and the foreign law application. Namely, by the Peace of Constance, the German Emperor recognized the autonomy of Italian cities. In the social circumstances of the time, the merchants from the cities of North Italy quite often made business deals with the merchants from other parts of the Empire (Flanders, the Hanseatic League cities etc.), so it was necessary to resolve disputes and apply customary rules applicable in one of those territories (​Pak, 1995, p. 219). The question of law application was also posed in the event of the conflict of rules between different cities, which occurred quite often due to spatial propinquity. Citing the authority of Roman law, the glossators created a dogma that enabled the application of the statute and/or law of another city. The dogma in the form of a gloss was first constructed by Accursius relying on the first constitution of the Code of Justinian that read: "Cunctos populos quos clementie nostre regit imperium..."3, from which a contrario conclusion was drawn that there are "those people over which the empire does not rule" and for which there are some other applicable regulations.

The dogma constituted the theoretical basis for the work of post-glossators, wellknown for being the first to create the collision method. The most outstanding post-glossators are Cynus, Bartolus de Sassoferrato and Baldus. Their method consisted of grouping the statutes which covered certain legal relationships. All statutes were grouped into three categories: real, personal and mixed (Živković, p. 17). The nature of the statute served as a criterion of their validity: the real ones were applied only in the territory of the city that had adopted them, the personal ones had an extraterritorial effect as well, while the mixed ones were applied to all legal matters concluded within the city (extraterritoriality existed for these statutes in case a dispute was resolved in the city different from the one in which the legal matter was originally concluded).

Particularity also characterized France in the 16^th century. The state was divided into several separate provinces, i.e. legal regions that constituted a sum of customs (coutimes) characteristic for each given region. On the other hand, the strengthening of bourgeoisie also meant resistance to the strict application of local customs and efforts for the broader application of Parisian customs, which also suited the monarch. We cannot speak of the conflict of laws and the foreign law application in this period the way we see them today. Just like in Italian cities, in France of the 16^th century we can speak of the conflict of the systems of rules, statutes and/or customs that cannot be equalized to the conflict of laws of different sovereignties. The names of Dumoulin and D'Argantré are important for the work of the French school. The affiliation to different social strata also marked the theoretical work of these writers. Dumoulin as a member of bourgeoisie strived for the application of the personality principle and is also known as the creator of the "autonomy of the will" concept. As a Breton baron, D'Argantré fought for the autonomy of Brittany and, accordingly, for the broader application of the territoriality principle.

The emergence of absolutist monarchies constituted the period of national unification and creation of a single national market. Some authors connect the establishment of private international law to this very period. Social relations that legally constitute the emergence of the conflict of laws between various sovereignties were created in this period. For the first time the conflict of laws arose between the domestic and the foreign country. Trade development, the intermediary role of the Netherlands in trade (17^th century) and the existence of colonial territories called for a different approach to the problem of the foreign law application. The legal basis for the foreign law application was found in the principle of "international comity" or "comity of nations" (comitas gentium). This principle was first mentioned by Dutch jurist Hugo Grotius. Grotius is considered to be the father of the newage school of natural law and the founder of public international law deprived of the religious influence. In his opinion, there is Divine law (ius divinum) deriving from Divine reason, and human law (ius humanum) in its three forms: natural law (ius naturae), civil law (ius civile) and international law (ius gentium) (Lukić, Košutić, 2006, p. 92). If Grotius is an important name in the theory of public international law, the Dutch-Flemish school of the time gave another outstanding name for the theory of private international law too. Namely, the starting principles for the foreign law application were established by Ulrik Huber. The third axiom in the chapter of his famous work Praelectiones iuris from 1689, which refers to the conflict of laws, allows the expansion of the effect of foreign law for reasons of courtesy. He believes that entire law is divided into: natural (ius naturae) and positive (ius voluntarium). Positive law is subdivided into: Divine (ius divinum) and human (ius humanum); human law is further divided into: civil (ius civile) and international law (ius gentium). Even at that time there was a pronounced attitude that ius gentium abstracted the principles referring to social necessities – the need for cohabitation (Zadorozhna, 2019, p. 161).

Comitas gentium

The original doctrine of the institute of comitas gentium is founded on the assumption of absolute territorial sovereignty of the state and its internal law (Đajić, 2006, p. 114). If foreign law is also applied, according to the teaching of the Dutch-Flemish school, it is done on the basis of the decision of a domestic court which accepts this possibility ex commodis et tacito populorum consenso (in line with the mutual interest and based on tacit agreement). Absorbing the achievements of the glossators and post-glossators, first the natural law school insisted on building the system and systemic manner of thinking, the central place in which was taken by the deductive manner of conclusion. Apart from the system of legal rules, there are also natural obligations and moral duties that regulate mutual behaviour. Huber's concept of comitas gentium is a set of rules the states take into account in their mutual relations based on tacit agreement. The foreign law application is not based on a legally binding rule. Although a strict positivist, Huber believed that the possibility of the foreign law application depended on the concerns of international comity, kindness and/or courtesy, and that the decision about the foreign law application constituted an act of goodwill of the states. Developing Huber's principle of "international comity", American theoretician Joseph Story believed that the real basis for the application of a foreign country's law lay in mutual interest and usefulness (Dornis, 2017, p. 4). Bringing the concept of comitas gentium closer to the legally binding rule was first confirmed in the decision of the US Supreme Court in the case Hilton v. Guyot (Joel, 2008, 27).

Comitas gentium is a rule that brings balance to social relations. Nowadays, comitas gentium is ranked as a principle of public international law imposing the obligation of cooperation and friendly relations between countries. Looking from the perspective of law application, this cooperation is definitely not a moral luxury or an extravagant deviation from the main course of resolving relations that are associated with more sovereignty. On the contrary, the possibility of the foreign law application in certain cases is the prerequisite for the stability of relations with the element of foreignness (Varadi et al., p. 105). It is a rule that may also be labelled as the rule of so-called "soft law".

Classical doctrines of the 19th and 20th centuries

The liberalist ideas of the French Bourgeois Revolution from 1789 triggered a series of events that constituted a complete break with the feudal system. On the other hand, the development of production forces led to the all-round international exchange of goods and services, which demanded the legal regulation of newly-formed relations. At the beginning of the 19^th century, first great codifications of the regulations were conducted in the field of civil law.

Theoretical trends created in this period may be classified into two concepts: universalist and positivist. Universalists thought that the regulation of the conflict of laws had to be uniform in all countries, which could be achieved by acting in line with the common model created a priori. The most important representative of this concept is definitely Savigny, a German writer from the historical law school whose a priori solution was focused on looking for solutions in a legal relationship that is associated with more sovereignty. Namely, according to Savigny, every legal relationship has its centre of gravity and seat (Sitz) and this very place towards which such relationship gravitates should serve as a connection point for determining the legal framework with the legal rules according to which the contentious legal relationship will be resolved. Just like Grotius and Huber, Savigny also thinks that the foreign law application may be labelled as friendly comity (comitas).

The period of the strengthening of national states also implied the strengthening of sovereignty and citizenship. As a representative of the universalist concept, Mancini strongly supported these attributes and based his solution to the conflict on the principle of lex nationalis. Every state should recognize the specific features of law, culture, customs and morality that bind a person to his/her national state (Batiffol, Lagarde, 1981, p. 268).

The third theory that offers a universalist solution is the theory of vested rights created by the renowned French theoretician Pillet. This theory finds a solution in the application of the principle of territoriality. In his work Pillet starts from two theses: about the permanence and generality of the application field of the regulations (Sajko, 2005, p. 37). According to Pillet, a paradox is possible at the moment of disrespect or, more precisely, impossibility to respect vested rights in the country in which such rights either do not exist or are contrary to its social order. The problem is primarily in the boundary, i.e. the measure reached by the restriction of the vested rights in the territory of the states obliged to observe it and to which those states should observe such rights, because they are bound to it by internationally accepted and established obligations towards other states (Ćorić, 93).

Positivist concepts do not search for universal solutions that would be recognized by all or the majority of countries. The essence of this concept was expressed by the Italian scholar Anzilotti, who believes that the rules of the conflict of laws are international in their effect, but formally internal (Pak, p. 223). The attitude that the norms about law application are of a strictly national character was advocated by French theoreticians Bartin and Kahn. Their works were a rebellion against the speculative teaching of the universalist school and the confusion it kept creating by the differences between the ideal model of collision norms and the actual status of positivist law in some countries (Živković, p. 73).

Unification of legal rules

Centrifugal forces had a strong effect in the territory of Europe at the beginning of this millennium. Economic, cultural and political unification at one moment also resulted in the idea of the necessary adoption of a unique constitution for Europe. This project failed in the ratification process at the 2004 referendum. The Treaty Establishing a Constitution for Europe was declined at the referendums in France (54.68% votes against, with the turnout of 69.34%) and the Netherlands (61.7% votes against, with the turnout of 63%) (Borhart, 2013, p.12). However, irrespective of the aspects of the political unification, the unification process of the private international law regulations in the territory of Europe proceeded more or less smoothly. In the stage of private law communitarization (legally ensured by the signing of the Treaty of Amsterdam in 1997), a series of regulations was adopted that unified collision norms in different fields of private law. The unification of collision regulations was conducted with the aim of achieving international harmony in decision-making, which implies that the application of unique collision rules will lead to equal solutions, i.e. the application of the legal regulations of the same legal framework to identical factual circumstances and situations. International decision-making harmony is a principle established by Savigny and it is also the ideal of private international law (Meškić, Đorđević, 2016, p. 25). This form of unification does not threaten the existence of private international law as a separate legal branch and it also takes care of the need for diversities existing between individual national systems of rules. However, the processes referring to the unification of material law rules, not only inside the EU, but also on a global scale, definitely do that.

One of the forms of regulating private legal relationships with the element of foreignness is also their direct regulation (in a direct manner, unlike the indirect collision method) by the unification of material law regulations that ultimately eliminates the conflict of laws and the need for their regulation. In historical terms, this method is not new. Ius gentium, i.e. law created by the praetors in the second half of the Republic period played a very important role in connecting nations with quite different cultures, religious and legal traditions (Bujuklić, p.80). Even nowadays there are similar, although definitely not the same circumstances, but what matters is which authority initiates these processes. An answer is imposed that legal certainty suits the centres of economic power when it comes to the exchange of goods and services on a global scale. Naturally, particularity of the solution to these matters is not desirable because it slows down international trade. However, a question arises whether this unification is justified and desirable when it comes to strictly personal rights of individuals.

Conclusion

To understand the relation between law and the society, it is necessary to understand the nature of law and legal development. According to Professor Watson, "the development of law is quite complex and one of very common factors in that development is borrowing or transplanting legal institutions, solutions and even systems". Law as a social upgrade depends on its base, i.e. the society that is also a dynamic category. Important theories of social changes and social development in sociology are cyclism, evolutionism and historicism (Šljukić, Šljukić, 2013, p. 474).

The reception of Roman law is proof in itself of dynamic and cyclic changes occurring in the field of law. Many institutions and institutes from Roman law are even today part of positive European-continental legal systems, although they are familiar in the Anglo-Saxon legal system too. The system of rules of ius gentium points to the roots of the reception of modern international principles. Moreover, comitas gentium as an evolved principle still constitutes the basis of the foreign law application today.

This rule is no longer considered only a gesture of goodwill through which one country applies the law of another country. It is an international obligation that is the foundation of international cooperation and that cannot be reduced to a simple basis of reciprocal behaviour by do ut des. In fact, those are complex and dynamic concessions made by countries to one another, while mutually observing legitimate interests in the application of other country's law and expecting that those countries will observe and respect its own legitimate interests in the domestic law application.

Although the foreign law application is seen as a civilization accomplishment, the paper also refers to integrative processes that appear in law in the form of the unification of legal rules. The emergence of economic globalization on a worldwide scale has already entered other spheres of social life. The globally increasing level of migrations has led to numerous contacts that do not penetrate only property and labour rights, but also other private rights of individuals (status, family, inheritance etc.). We believe that erasing differences as a reflection of the century-old cultural identity of certain nations, is unjustified and undesirable and that diversity should be pursued both in modern law and society.