A union of states is there for various reasons but a fundamental one is to provide public goods that cannot be provided optimally by actions at the level of individual states. Such union-level public goods would be, for example, defence of the union against aggression by other states from outside it, protection against infectious diseases spreading in the union, regulation of massive population flows into the union, and safeguarding of financial and macroeconomic stability of the union. These kinds of goods are characterised by two important qualities so that they can be classified as public goods at the level of a union of states: ‘nonrivalness’ and ‘nonexcludability’. These concepts have been elementary concepts of economics since the work of the renowned economist Paul Samuelson in the 1950s.