 The main problem with international law, according to Hart, is that it lacks secondary rules. This means it is not law in a modern developed sense, but in a simple or primitive one. In Hart’s well-known conception, a legal system is a union of two sets of rules: primary rules which impose obligations, and secondary rules which are rules about rules, relating to the way in which those primary rules are identified, changed and adjudicated  upon.[2]  Hart argues that international law possesses primary rules which are for the most part observed in practice. International law however lacks that distinctive secondary rule which Hart calls the the rule of recognition, that is a rule that does not depend for its existence on other rules and provides the ultimate foundation on which the legal system depends.