The history of political thought dates back to antiquity while the history of the world and thus the history of political thinking by man stretches up through the Medieval period and the Renaissance. In the Age of Enlightenment, political entities expanded from basic systems of self-governance and monarchy to the complex democratic and communist systems that exist of the Industrialized and the Modern Era. In parallel, political systems have expanded from vaguely defined frontier-type boundaries, to the definite boundaries existing today. The history of political thought has often overlapped with the history of philosophy.