Contrary to the views expressed by Constantinesco, Kokkini postulates the suitability of comparative concepts for direct use in the initial phase of comparative research. (7) Comparative concepts are important since they prevent a waste of time in a later phase of research. Testing the comparability assumption must take place as soon as possible to exclude disappointments after visiting many libraries in different countries in order to study information which proves to be serve as unequivocal starting points, which must be upheld in later phases of comparison ncepts irrelevant because of the absence of comparability. According to Kokkini, comparative concepts The controversy between Constantinesco and Kokkini does not decisively assess the relative usefulness of extensional concepts since Kokkini does not explicitly reject the empirical use of extensional concepts. Therefore, I have developed the following arguments against extensional concepts: national terms cannot identify the common elements of, e.g., 'apartment ownership. The intersection containing the common elements can only be identified if the common characteristics of these rules are known. Rules are at the intersection of different legal systems if and only if they ave the characteristics included in the common intension of the comparative concept. As will be shown, extensional concepts do not comply with the conditions explained in subsections 1. 1 and 1.2, stating that the intension of the comparative concept must be explicit and unequivocal The juxtaposition of legal rules, e.g. those of the German 'Diebstahl and of the English theft' mentioned in subsection 2. 1 above, is not sufficient to conclude that common elements are present. After all, the national terms Diebstahl"and theft only refer to German and Engli respectively, without relating these rules. A listing of national terms does not determine the intension to establish that property' and appropriation are two similarities justifying the comparability of the German 'Diebstahl" and the English theft. The terms 'unlawfully' and dishonestly could also belong to the common elements. The comparatist cannot objectively exclude these elements as being purely national elements without using the criteria that goods must be moveable and that it must be possible for another person to own them. The comparatist eds a uniform perspective like that of'co-ownership' in the context of 'apartment ownership described in subsection 1.2. Terminological resemblance does not guarantee any conceptual correspondence fixing the comparability of the rules of different legal systems. Extensional concepts do not necessarily provide observable criteria that enable us to investigate empirically the rules belonging to different legal systems Extensional concepts do not justify the comparability of legal rules. They do not make understandable which characteristics of foreign legal rules account for their comparability. The legal rules to be compared do not provide their own comparability. Comparability is not a given fact which is without doubt. Therefore, it has to be discussed which concepts are useful to compare legal rules: functional concepts on the one hand, or immanent concepts on the other 3.2 Functional conceptsContrary to the views expressed by Constantinesco, Kokkini postulates the suitability of comparative concepts for direct use in the initial phase of comparative research.(7) Comparative concepts are important since they prevent a waste of time in a later phase of research. Testing the comparability assumption must take place as soon as possible to exclude disappointments after visiting many libraries in different countries in order to study information which proves to be irrelevant because of the absence of comparability. According to Kokkini, comparative concepts serve as unequivocal starting points, which must be upheld in later phases of comparison. The controversy between Constantinesco and Kokkini does not decisively assess the relative usefulness of extensional concepts since Kokkini does not explicitly reject the empirical use of extensional concepts. Therefore, I have developed the following arguments against extensional concepts: national terms cannot identify the common elements of, e.g., 'apartment ownership'. The intersection containing the common elements can only be identified if the common characteristics of these rules are known. Rules are at the intersection of different legal systems if and only if they have the characteristics included in the common intension of the comparative concept. As will be shown, extensional concepts do not comply with the conditions explained in subsections 1.1 and 1.2, stating that the intension of the comparative concept must be explicit and unequivocal. The juxtaposition of legal rules, e.g. those of the German 'Diebstahl' and of the English 'theft' mentioned in subsection 2.1 above, is not sufficient to conclude that common elements are present. After all, the national terms 'Diebstahl' and 'theft' only refer to German and English rules, respectively, without relating these rules. A listing of national terms does not determine the intension to establish that 'property' and 'appropriation' are two similarities justifying the comparability of the German 'Diebstahl' and the English 'theft'. The terms 'unlawfully' and 'dishonestly' could also belong to the common elements. The comparatist cannot objectively exclude these elements as being purely national elements without using the criteria that goods must be moveable and that it must be possible for another person to own them. The comparatist needs a uniform perspective like that of 'co-ownership' in the context of 'apartment ownership' described in subsection 1.2. Terminological resemblance does not guarantee any conceptual correspondence fixing the comparability of the rules of different legal systems. Extensional concepts do not necessarily provide observable criteria that enable us to investigate empirically the rules belonging to different legal systems. Extensional concepts do not justify the comparability of legal rules. They do not make understandable which characteristics of foreign legal rules account for their comparability. The legal rules to be compared do not provide their own comparability. Comparability is not a given fact which is without doubt. Therefore, it has to be discussed which concepts are useful to compare legal rules: functional concepts on the one hand, or immanent concepts on the other. 3.2 Functional concepts