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He argues that his position shows "that contract, predicated on private property [can] reach to the furthest realms of human interaction, even to voluntary slave contracts." [ "Towards a Libertarian Theory of Inalienability: A Critique of Rothbard, Barnett, Smith, Kinsella, Gordon, and Epstein," pp. 39-85, Journal of Libertarian Studies , vol. 17, no. 2, p. 44, p. 48, p. 82 and p. 46] So the logic is simple, you cannot really own something unless you can sell it.