I ought to mention the type of liberty that I assume libertarians to be defending. It is a kind of interpersonal liberty. In particular, it is about people being unconstrained by other people’s interferences, or invasions, or aggressions, or trespasses, or—as I prefer to theorize it—it is about the absence of proactively imposed costs, ultimately in a prepropertarian sense.

J. C. Lester - A Critical Commentary on Walter Block's "David Friedman and Libertarianism: A Critique" and a Comparison with J. C. Lester's Responses to Friedman