The name that comes to mind is John Locke. Jefferson, Madison and other founding fathers pointed to him most of all. I would point out that it was Rousseau who penned what some would consider the classic treatise on contract theory, liberal republicanism and defining the sovereign separate from the state. And surely Leviathon, a book by Thomas Hobbes, talking about the necessity of representative government and the equality of all men was certainly in Jefferson's library. But the easy answer is John Locke but certainly not the all inclusive answer.
