Quick answers first, details follow:
BARON DE MONTESQUIEU - checks and balances
VOLTAIRE - importance of religious tolerance
JOHN LOCKE - natural rights
The "Separation of Powers" principle (or "checks and balances," as we often refer to it), was introduced by Montesquieu in an important work of political theory called The Spirit of the Laws, published in 1748. Within his treatment of how governments will function best, Montesquieu argued that executive, legislative, and judicial functions of government ought to be divided between parts of the government, so that no one person or division of the government can infringe on the overall rights of others in the government or of the members of the society overall.
During the period of the Wars of Religion in the 16th and 17th centuries, ideas of religious tolerance began to develop among a group of thinkers known as the politiques in France, who believed the stability of society is a greater concern than private matters of faith. During the period of the Enlightenment in the 18th century, the philosopher, novelist and playwright Voltaire wrote strongly in support of religious tolerance.
John Locke was one of the first of the Enlightenment era philosophers. The Enlightenment's emphasis on reason was in contrast to superstition and traditional beliefs. The Scientific Revolution had shown that there are natural laws in place in the physical world and in the universe at large. Applying similar principles to matters like government and society, using reason will guide us to the best ways to operate politically so as to create the most beneficial conditions for society. This included a conviction that all human beings have certain natural rights which are to be protected and preserved.
