Baron Charles de Montesquieu was an extremely influential French political philosopher who lived between 1689 and 1755.
At only the age of 27, he anonymously published Persian Letters in which he "mocks the [ended] reign of Louis XIV" and "pokes fun at all social classes"; he even refers to the theories of Thomas Hobbes. The work was extremely successful, and though published anonymously, his identity was soon discovered, and the work made him famous. From there, he became a member of Parlement in Bordeaux.
Montesquieu's most influential work was The Spirit of Laws, published, again anonymously, in 1748. In it, Montesquieu uses anthropology to derive new classifications for forms of government that his predecessors had identified as "monarchy, aristocracy, and democracy." Instead, he redefines government forms based on the "government's manner of conducting business," not based on how a government's power is distributed. For example, he defines a republic as acting based on virtue and a despotism as acting based on fear (Encyclopaedia Britannica, "Montesquieu: Major works"). Drawing on arguments from John Locke, he also argues for the separation of powers. He also argued for civil rights, such as the "right to a fair trial" ("The Spirit of the Law").