The controversy between Zweigert/Kotz and Constantinesco helps to settle the relative usefulness of extensional concepts since Zweigert and Kotz strongly subscribe to the applicability of functional concepts, whereas Constantinesco rejects their functional approach According to Zweigert and Kotz, (8)a social function is the common perspective the researcher needs. They state that rules of different legal systems can be compared if they serve the same function. In their view, the legal rules of every society essentially face the same problems. As Zweigert and Kotz are sceptical of the conceptual constructs of particular nations, complete abstraction from national concepts is to be achieved with the help of functional concepts Conceptual systems would only generate some order in national law. Thus instead of asking What formal requirements are there for sales contracts in foreign law?', they prefer to ask 'How does foreign law protect parties from surprise? Constantinesco rejects the standpoint held by Zweigert and Kotz (9)His criticism can be illustrated by focusing on the phrasing 'How does foreign law protect parties from surprise? In this exemplary phrasing, Zweigert and Kotz omit the legal concepts of formal requirements and of sales contracts' in the initial phase of comparative research. According to Constantinesco, they do not make a distinction between the various legal problems which have been specified by national rules. His criticism can be summarized in the following way: as concepts for describing legal rules, Zweigert and Kotz's functional concepts are too broad and ill-defined. However, Zweigert and Kotz do not answer Constantinesco' s criticism. No discussion between these scholars has been found in the literature The following arguments may complete the criticism by Constantinesco: in order to test the comparability assumption containing the functional concept, the causal relation between legal rules and the resolution of a social problem needs to be established. The social effect of legal rules is difficult to determine, however, so in most cases Zweigert and Kotz's functional concepts have no empirical use. A further explanation will be given to clarify these additional arguments he formation of functional concepts occurs in relation to social problems such as housing shortage, to which, for instance, the rules of apartment ownership are solutions assuming that the legislator cares about the building of apartments. Consequently, the comparatist has to investigate the societal impact of different sets of rules of 'apartment ownership. Specifically, he has to investigate every desirable or undesirable effect of the legal rules of apartment ownership. Such rules may promote the conforming behaviour of investors in one society, but may not have the same positive consequences in a different society because of impediments to the laws effective functioning. Illustrating one of the most important impediments, some ignorance of the law may intervene between the promulgation of the law and the behaviour of potential investors. The rate of conforming behaviour may vary greatly in the societies under investigation; this circumstance turns the formation of functional concepts into an almost arbitrary decision. Besides, the comparatist is not limited to those rules legally defined as rules of apartment ownership' since according to Zweigert and Kotz, the starting point of comparison should not be found in law itself. If different rules may solve the social problem of housing shortage, the comparatist has to expand his research to functional equivalents such as building regulationsThe controversy between Zweigert/Kötz and Constantinesco helps to settle the relative usefulness of extensional concepts since Zweigert and Kötz strongly subscribe to the applicability of functional concepts, whereas Constantinesco rejects their functional approach. According to Zweigert and Kötz,(8) a social function is the common perspective the researcher needs. They state that rules of different legal systems can be compared if they serve the same function. In their view, the legal rules of every society essentially face the same problems. As Zweigert and Kötz are sceptical of the conceptual constructs of particular nations, complete abstraction from national concepts is to be achieved with the help of functional concepts. Conceptual systems would only generate some order in national law. Thus instead of asking, 'What formal requirements are there for sales contracts in foreign law?', they prefer to ask 'How does foreign law protect parties from surprise?'. Constantinesco rejects the standpoint held by Zweigert and Kötz.(9) His criticism can be illustrated by focusing on the phrasing 'How does foreign law protect parties from surprise?' In this exemplary phrasing, Zweigert and Kötz omit the legal concepts of 'formal requirements' and of 'sales contracts' in the initial phase of comparative research. According to Constantinesco, they do not make a distinction between the various legal problems which have been specified by national rules. His criticism can be summarized in the following way: as concepts for describing legal rules, Zweigert and Kötz's functional concepts are too broad and ill-defined. However, Zweigert and Kötz do not answer Constantinesco's criticism. No discussion between these scholars has been found in the literature. The following arguments may complete the criticism by Constantinesco: in order to test the comparability assumption containing the functional concept, the causal relation between legal rules and the resolution of a social problem needs to be established. The social effect of legal rules is difficult to determine, however, so in most cases Zweigert and Kötz's functional concepts have no empirical use. A further explanation will be given to clarify these additional arguments. The formation of functional concepts occurs in relation to social problems such as housing shortage, to which, for instance, the rules of 'apartment ownership' are solutions assuming that the legislator cares about the building of apartments. Consequently, the comparatist has to investigate the societal impact of different sets of rules of 'apartment ownership'. Specifically, he has to investigate every desirable or undesirable effect of the legal rules of 'apartment ownership'. Such rules may promote the conforming behaviour of investors in one society, but may not have the same positive consequences in a different society because of impediments to the law's effective functioning. Illustrating one of the most important impediments, some ignorance of the law may intervene between the promulgation of the law and the behaviour of potential investors. The rate of conforming behaviour may vary greatly in the societies under investigation; this circumstance turns the formation of functional concepts into an almost arbitrary decision. Besides, the comparatist is not limited to those rules legally defined as rules of 'apartment ownership' since, according to Zweigert and Kötz, the starting point of comparison should not be found in law itself. If different rules may solve the social problem of housing shortage, the comparatist has to expand his research to functional equivalents such as building regulations