The existence — and even the definition — of war in humanity's hypothetical state of nature has been a controversial topic in the history of ideas at least since Thomas Hobbes in "Leviathan" (1651) argued a "war of all against all", a view directly challenged by Jean-Jacques Rousseau in a "Discourse on Inequality" (1755) and "The Social Contract" (1762). The debate over human nature continues, spanning contemporary anthropology, archaeology, ethnography, history, political science, psychology, primatology, and philosophy in such divergent books as Azar Gat's "War in Human Civilization" and Raymond C. Kelly's "Warless Societies and the Origin of War". For the purposes of this article, "prehistoric war" will be broadly defined as a state of organized lethal aggression between autonomous communities.