“The only proper role of government, according to libertarians, is that of the protector of the citizen against aggression by other individuals. The government, of course, should never initiate aggression; its proper role is as the embodiment of the retaliatory use of force against anyone who initiates its use.”

America's Drug Forum interview (1991)
Context: The proper role of government is exactly what John Stuart Mill said in the middle of the 19th century in On Liberty. The proper role of government is to prevent other people from harming an individual. Government, he said, never has any right to interfere with an individual for that individual's own good.
The case for is exactly as strong and as weak as the case for prohibiting people from overeating. We all know that overeating causes more deaths than drugs do. If it's in principle OK for the government to say you must not consume drugs because they'll do you harm, why isn't it all right to say you must not eat too much because you'll do harm? Why isn't it all right to say you must not try to go in for skydiving because you're likely to die? Why isn't it all right to say, "Oh, skiing, that's no good, that's a very dangerous sport, you'll hurt yourself"? Where do you draw the line?

On the United States Declaration of Independence, and the United States Constitution at the U.S. Bicentennial celebrations, Independence Hall, Philadelphia, Pennsylvania (4 July 1976)
1970s
Context: The Declaration was not a protest against government but against the excesses of government. It prescribed the proper role of government to secure the rights of individuals and to effect their safety and their happiness. In modern society, no individual can do this all alone, so government is not necessarily evil but a necessary good.
The framers of the Constitution feared a central government that was too strong, as many Americans rightly do today. The framers of the Constitution, after their experience under the Articles, feared a central government that was too weak, as many Americans rightly do today. They spent days studying all of the contemporary governments of Europe and concluded with Dr. Franklin that all contained the seeds of their own destruction. So the framers built something new, drawing upon their English traditions, on the Roman Republic, on the uniquely American institution of the town meeting.

Part One, chapter 4, page 18
Why Government Doesn't Work (1995)

Fifth Lecture, Applications in Statistics and the Theory of Errors, p. 159
Probability, Statistics And Truth - Second Revised English Edition - (1957)

Journals of Ayn Rand (1997)

Zero Aggression Principle ("ZAP"), from "Who is a Libertarian?"
Variant: A libertarian is a person who believes that no one has the right, under any circumstances, to initiate force against another human being, or to advocate or delegate its initiation. Those who act consistently with this principle are libertarians, whether they realize it or not. Those who fail to act consistently with it are not libertarians, regardless of what they may claim.

The Supreme Court: How it Was, How it Is (1987).
Books, articles, and speeches
Context: An oft-heard description of the Supreme Court is that it is the ultimate protector in our society of the liberties of the individual. This phrase describes an important role of the Supreme Court, but by ignoring other equally important functions of the Court, it has a potential for mischief. It is a fairly short leap from this language to a feeling that the US Constitution is somehow "vindicated" every time a claim of individual right against government is upheld, and is not vindicated whenever such a claim is not upheld. But this, of course, cannot be the case. The role of the Supreme Court is to uphold those claims of individual liberty that it finds are well-founded in the Constitution, and to reject other claims against the government that it concludes are not well-founded. Its role is no more to exclusively uphold the claims of the individual than it is to exclusively uphold the claims of the government: It must hold the constitutional balance true between these claims.

The Virtue of Selfishness (1964)

Source: Nature of Man and His Government (1959), p. 73