Truth.

Stuart, Portrait of Thomas Jefferson, 1805

Reason and experiment have been indulged, and error has fled before them. It is error alone which needs the support of government. Truth can stand by itself. Subject opinion to coercion: whom will you make your inquisitors? Fallible men; men governed by bad passions, by private as well as public reasons. And why subject it to coercion? To produce uniformity. But is uniformity of opinion desirable? No more than of face and stature.

Thomas Jefferson, Notes on the State of Virginia: Query 17