Though much of Enlightenment political thought was dominated by social contract theorists, both David Hume and Adam Ferguson criticized this camp. Hume's essay Of the Original Contract argues that governments derived from consent are rarely seen, and civil government is grounded in a ruler's habitual authority and force. It is precisely because of the ruler's authority over-and-against the subject, that the subject tacitly consents; Hume says that the subjects would "never imagine that their consent made him sovereign", rather the authority did so. Similarly, Ferguson did not believe citizens built the state, rather polities grew out of social development. In his 1767 An Essay on the History of Civil Society, Ferguson uses the four stages of progress, a theory that was very popular in Scotland at the time, to explain how humans advance from a hunting and gathering society to a commercial and civil society without "signing" a social contract.

Answer this question, if possible (if impossible, reply "unanswerable"): In what year did Ferguson publish An Essay on the History of Civil Society?
1767