With regard to history, the most important principle that came out of the Enlightenment was the idea of democracy. Of course, this was not a new idea, but the Enlightenment created a climate in which there was much more support for the idea. Enlightenment thinkers like John Locke contributed ideas that led directly to the American Revolution and the democracy that was set up in the wake of that war.
A major idea of the Enlightenment was that of rationalism. This held that truth should be found through reason, not through blind adherence to tradition. This idea helped lead to the principle of self-government. Enlightenment thinkers argued that hereditary monarchies were not based on any sort of rational ideas but were instead based on the irrational idea of divine right of kings. This led them to the principle that legitimate government could only be based on the consent of the governed.
This was, historically speaking, perhaps the most important principle derived from the Enlightenment.