Public goods theory is based on some of the more obscure logic of political economics, based on the concept that a 'public good' is an exception to the 'free' market economy. This theory holds that a public good is 'consumed for free' by the entire society, and cites law enforcement, defense, and fire departments as common cases of public goods. It also states that these goods are 'non-exclusive', i.e. available to all. The concept has few if any practical applications in the real world, but is used as a basis for economic rationales.