posted by Christopher .
is core to the notion of “life, liberty and the pursuit of happiness” in the Declaration of Independence
the idea that government may never restrict citizens’ liberty
I don't think so. The government can restrict our liberty in certain situations. For instance we can't drive 50 mph in a 30 mph zone.
The meaning of those words can be interpreted in many ways. A good explanation of the roots of the idea is here:
the concept of American exceptionalism