Baron de Montesquieu
Montesquieu was a French political philosopher (1689-1755), very famous for popularizing the separation of powers, in his work The Spirit of the Laws (1748), a treatise on political theory.
In the treatise, the philosopher describes the three types of government: democracy, republic, monarchy and despotism, and supported the separation of powers into three branches: legislative, executive, and judiciary branch which should rule under the law and check each other’s power, preventing from corruption.
This theory set the base for many of the actual governments.