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The Lockean proviso is a feature of John Locke's labour theory of property which states that, whilst individuals have a right to homestead private property from nature by working on it, they can do so only "...at least where there is enough, and as good, left in common for others".
Nor was this appropriation of any parcel of land, by improving it, any prejudice to any other man, since there was still enough and as good left, and more than the yet unprovided could use. So that, in effect, there was never the less left for others because of his enclosure for himself. For he that leaves as much as another can make use of, does as good as take nothing at all. Nobody could think himself injured by the drinking of another man, though he took a good draught, who had a whole river of the same water left him to quench his thirst. And the case of land and water, where there is enough of both, is perfectly the same.— Second Treatise of Government, Chapter V, paragraph 33
The phrase "Lockean proviso" was coined by libertarian political philosopher Robert Nozick in Anarchy, State, and Utopia. It is based on the ideas elaborated by John Locke in his Second Treatise of Government—that self-ownership allows a person the freedom to mix his or her labor with natural resources, thus converting common property into private property. Locke concludes that people need to be able to protect the resources they are using to live on, their property, and that this is a natural right. Nozick used this idea to form his Lockean proviso which governs the initial acquisition of property in a society. But in order for his ideas of ownership of property to get off the ground and be cogent, he devised the criterion to determine what makes property acquisition just, which is the proviso. The proviso says that though every appropriation of property is a diminution of another's rights to it, it is acceptable as long as it does not make anyone worse off than they would have been without any private property. 
Locke's proviso has been used by Georgists and socialists to point to land acquisition as illegitimate without compensation. In Georgism, the possession of land is proper only so long as the market rent is paid to the relevant community. If a plot of land has a positive rent, that implies that there is not land of similar quality freely available to others.
- Nozik, Robert (1971). Anarchy, State, and Utopia. p. 175.
- Rothbard, Murray (2003). The Ethics of Liberty. NYU Press.