John Locke justified the existence of property this way: (God gave) the earth to all mankind, to have dominion over in common and equally. We therefore have a "state of nature" without private property.
However, if a man clears a forest and makes a farm, he combines his labour with nature to make something not entirely natural. The result is his property, by right of his work. Locke says that this right of property should be within limits...he should be able to control enough property to sustain himself, and just enough that he can work it personally. Locke believed this degree of private property is “fair” (i.e. it is justified by natural rights).
Karl Marx points out that Locke's "fair" definition of private property is simply not how property is defined and used in our society. The ownership of land (or any other “means of production”) does not imply that one works the land. In fact, most of the time people’s labour is alienated from them; they labour, and the benefits go to the owner, not themselves. Marx believes this is “unfair” (i.e. it is not justified by natural rights).
Locke says that human beings have the right to resist if an individual tries to take their natural rights away, and to revolt if a government tries to take them away.