3. Separation of powers
Baron de 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), which is a treatise on political theory.
In the treatise, the philosopher describes three types of government: democracy, republic, monarchy and despotism; and he supported the separation of powers into three branches: legislative, executive, and judicial which should rule under the law and must check each other’s power in order to prevent from corruption or abuses of power.