The formation of extensional concepts is the listing of common elements which may be present in several legal systems. These common elements are to be found at the intersection of different sets of legal rules, or parts of rules, belonging to different systems. In this view, the national sets of rules could be identified by means of legal terms, e.g. the Dutch 'appartementsrecht', the German Wohnungseigentum'and the French 'copropriete des immeubles batis. Without choosing any intension for the comparative concept, the extensional concept of ' apartment ownership refers to the common elements at the intersection The following example will illustrate the formation of extensional concepts. The comparatist may ahl"and of English theft, respectively: W takes moveable property not his own from another with the intention of unlawfully appropriating it to himself shall be punished.. (5)and: 'A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it. 'The first rule has been identified by the german legal term Diebstahl and the second one by the term theft in the English Theft Acts. Further, the common elements have to be found at the intersection of these rules. The comparatist may decide that 'property' and'appropriation' are common elements, intuitively excluding unlawfully'and'dishonestly'. He may conclude that the German Diebstahl and the English theft are comparable since extensional concepts result from the listing of common elements 2.2 Functional concepts The formation of functional concepts occurs in relation to social problems to which legal rules are a solution. In this view, the starting point for comparison is not to be found in law itself but in social problems. For instance, different rules of full adoption and of limited adoption will be regarded as a means of solving the problems of neglected children. Functional concepts arise from the perspective of social science. This perspective is external to the legal systems under comparison. Correspondingly, the complete abstraction from national concepts must be achieved Functional concepts are expected to refer to legal rules although functional concepts should be completely independent of legal concepts Functional concepts are related to factual needs or problems of different societies. In this view, the legal rules of different systems would produce social effects on human behaviour which, in its turn, is expected to meet the needs of societies. Correspondingly, the comparatist has to identify common needs or common problems and he has to determine which legal rules are solutions to these common problems. Legal rules considered to be solutions justify the formation of the functional concept. These premises underlying the functional approach lead to the starting point for comparison: the functional concept, e.g. 'apartment ownership, permits the comparison of rules of different legal systems in respect of similar needs or problems such as housing shortage 2.3 Immanent conceptsThe formation of extensional concepts is the listing of common elements which may be present in several legal systems. These common elements are to be found at the intersection of different sets of legal rules, or parts of rules, belonging to different systems. In this view, the national sets of rules could be identified by means of legal terms, e.g. the Dutch 'appartementsrecht', the German 'Wohnungseigentum' and the French 'copropriété des immeubles bâtis'. Without choosing any intension for the comparative concept, the extensional concept of 'apartment ownership' refers to the common elements at the intersection. The following example will illustrate the formation of extensional concepts. The comparatist may juxtapose the following rules of German 'Diebstahl' and of English 'theft', respectively: 'Whoever takes moveable property not his own from another with the intention of unlawfully appropriating it to himself shall be punished ...',(5) and: 'A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it.' The first rule has been identified by the German legal term 'Diebstahl' and the second one by the term 'theft' in the English Theft Acts. Further, the common elements have to be found at the intersection of these rules. The comparatist may decide that 'property' and 'appropriation' are common elements, intuitively excluding 'unlawfully' and 'dishonestly'. He may conclude that the German 'Diebstahl' and the English 'theft' are comparable since extensional concepts result from the listing of common elements. 2.2 Functional concepts The formation of functional concepts occurs in relation to social problems to which legal rules are a solution. In this view, the starting point for comparison is not to be found in law itself but in social problems. For instance, different rules of 'full adoption' and of 'limited adoption' will be regarded as a means of solving the problems of neglected children. Functional concepts arise from the perspective of social science. This perspective is external to the legal systems under comparison. Correspondingly, the complete abstraction from national concepts must be achieved. Functional concepts are expected to refer to legal rules although functional concepts should be completely independent of legal concepts. Functional concepts are related to factual needs or problems of different societies. In this view, the legal rules of different systems would produce social effects on human behaviour which, in its turn, is expected to meet the needs of societies. Correspondingly, the comparatist has to identify common needs or common problems and he has to determine which legal rules are solutions to these common problems. Legal rules considered to be solutions justify the formation of the functional concept. These premises underlying the functional approach lead to the starting point for comparison: the functional concept, e.g. 'apartment ownership', permits the comparison of rules of different legal systems in respect of similar needs or problems such as housing shortage. 2.3 Immanent concepts