On the difficulties regarding the formation of customary law in the field of taxation

This chapter deals with the issue of the existence and the process of establishing international customary law in the field of taxation. For the purpose of the present analysis, reference is made to the traditional concept of international custom, consisting in a uniform and constant practice of states (diuturnas) coupled with the awareness and knowledge of the obligation to follow that practice (opinion juris sive necessitatis). Within the broad spectrum of its different types and characteristics, first it has to be noted that the international customs regarding tax matters fall into the category of declaratory customary law that facilitates the coexistence and cooperation between states. The existence of these rules, in fact, is ascertained through a process that considers the behaviour of states in order to identify the norms belonging to the field of general international law. In this respect, they differ from ‘‘moral’‘ rules (for example, the prohibition of the use of force, or human rights), which have as a starting point an idealistic model and are oriented towards future behaviours, so that great importance is attached to the diuturnas in the process of ascertaining whether they constitute rules of customary international law? Although the existence of tax norms with a kind of ‘‘.moral’’ content cannot be excluded (for example environmental taxation), such norms play a marginal role for the purpose of the present analysis.