Final answer:
Montesquieu believed in the separation of government powers into different branches to preserve political liberty, a principle reflected in the U.S. Constitution.
Step-by-step explanation:
The Enlightenment thinker who believed it was crucial that government be separated into different branches was Montesquieu. He argued in his work The Spirit of the Laws that to maintain political liberty, the powers of government should be divided among distinct entities.
Specifically, Montesquieu advocated for the separation of the executive, legislative, and judicial functions of government to ensure that no single branch could overextend its power, with each branch having the ability to restrain the others.
This concept is now a fundamental element of modern democratic systems, most notably evident in the United States Constitution.