Possession and control of an object enhance the freedom of its holder. Property protects this freedom; but it restricts the freedom of ail others. Drawing the boundaries of freedom with respect to external objects is a central and difficult challenge for libertarians. What justifies my ownership of, say, the cup I am drinking from? Several possible answers come to mind: answers based on need, on welfare, on desert or on equality. None of these is endorsed by the libertarian; none of these, arguably, can justify private property in the cup. From a proprietarian point of view the legitimacy of any holding derives from the moral power of its previous owner. An owner has the power of transfer, the power to make another person the owner. What justifies my ownership of the cup is the exercise of the power of the person who gave it to me; his ownership was justified by the exercise of the power of the person who sold it to him; the latter's ownership was justified by the exercise of the power of the person who bequeathed it to him; and so on and so forth. But series of transfers of this kind, long as they may be, must come to an end. There must have been a point where something not privately owned became the private property of an individual or group.
Libertarians must assert a principle of just appropriation. Such a principle specifies the ways in which a person can come to own a natural resource which was previously not (privately) owned.