Robert Nozick, 1938-2002, was a professor at Harvard whose best known book is Anarchy, State, and Utopia (1974) – a libertarian answer to Rawls’ A Theory of Justice (1971).
Most controversially, Nozick argued that a consistent upholding of the non-aggression principle would allow and regard as valid consensual or non-coercive enslavement contracts between adults. He rejected the notion of inalienable rights advanced by Locke and most contemporary capitalist-oriented libertarian academics, writing in Anarchy, State and Utopia that the typical notion of a "free system" would allow adults to voluntarily enter into non-coercive slave contracts.
Murray Rothbard, 1926-1995, wrote The Ethics of Liberty as his main political philosophy work. He accepted the labor theory of property, arguing that mixing labor with unowned land made the land private property which could then trade hands by trade or gift. He rejected the Lockean proviso that individuals could only homestead land where “there is enough, and as good, left in common for others.”
Rothbard was concerned with how we know what is right or good. His is Aristotle’s natural law reasoning. He rejected Mises conviction that ethical values remain subjective. Rothbard concludes that interventionist policies do benefit some people, including certain government employees and welfare beneficiaries.
Lecture 10 of 10 from David Gordon's The History of Politcal Philosophy: From Plato to Rothbard.