 In contemporary economics, goods are usually defined as public goods if and only if they are both non-rivalrous and non-excludable (e.g., Varian 1992: 414). Rivalrous and excludable goods are called private goods. National defence is a paradigmatic example of a public good. Not only does Sally’s consumption of national defence not reduce Bob’s consumption; she could not prevent him from consumption if she tried. Food, clothes and flats are paradigmatic examples of private goods.