As proponents of free-market anarchism, Benjamin Tucker and Gustave de Molinari first explicitly proposed private defense agencies. The concept later was advanced and expanded upon by anarcho-capitalists who consider the state to be illegitimate and therefore believe defense is something that should be provided or determined privately by individuals and firms competing in a free market. The Mises Institute published a book of essays entitled '. This concept is similar to polycentric law. Within economics, discussion of the concept largely has been confined to the Austrian School, as in Hans Hoppe's article "The Private Production of Defense" published by the Mises Institute.