Montesquieu was the famous Enlightenment thinker known for the concept of the separation of powers in government. In his influential work "The Spirit of the Laws," Montesquieu argued that political power should not be concentrated in one entity but divided among separate branches—executive, legislative, and judicial. This framework was designed to prevent tyranny and ensure that each branch would serve as a check on the others. His ideas profoundly influenced the development of modern democratic systems, particularly the structure of the United States Constitution, contributing to the foundational principles of liberty and balanced governance that still resonate today.