According to most thinkers of the Enlightenment, the source of government legitimacy is the consent of the people. This can be seen in the Declaration of Independence where Thomas Jefferson, who was influenced very strongly by Enlightenment thinkers, said that
Governments are instituted among Men, deriving their just powers from the consent of the governed…
This is, to Enlightenment thinkers, the only proper source of legitimacy.
Before the Enlightenment, governmental legitimacy tended to come from tradition, religion, and force. Governments were run by monarchs who got their legitimacy from the fact that their families had ruled for generations and from the idea that God intended that their families should rule. In addition, there was much more of an idea in those days that might made right and that military power equaled legitimacy.
Enlightenment thinkers argued that these were not valid sources of legitimacy. They believed that people created governments to protect their rights. People entered into a “social contract” in which they allowed themselves to be ruled in order to have their basic rights protected. Thus, government was legitimate because the people agreed to be governed. This was the source of legitimacy in the eyes of Enlightenment thinkers.