According to John Locke, private property is a natural right because the ownership of things is the only means by which a person can sustain himself or herself in physical comfort. Even though the natural condition of everything on earth and in it is that of common ownership, without a prior personal claim by any human being, people cannot make use of any of these things unless a certain method of appropriation is utilized.
This method of appropriation, according to Locke, is labor. The definition of labor in this case is basic: if a person applies any kind of natural, lawful effort to gain possession of something that was previously considered common property - in other words, without an individual owner - this thing becomes this person's private property. At this point, no other person can take possession of the said thing without the original owner's consent.
Locke distinguishes between the natural and unnatural actions seeking to gain possession of something, with it depending on whether the something sought for acquisition is already in private possession or still within the common domain. A person who seeks the goods already in possession of someone else does not really seek to apply his labor to the acquisition of the said goods, but rather to enjoy the fruits of someone else's labor without giving anything in return.
Should not a person taking possession of something in such a manner first ask permission of others, considering that the said something has been previously in common possession? Locke shows the impracticality of such an approach by a simple example of a hungry person having to ask all the people in the world whether or not he can eat an apple he himself plucked from a tree or picked up underneath it. In the same manner, if a single person takes possession of something while leaving a plentiful supply of the same thing for others to use, no usurpation of property occurs, and such action must be considered a natural occurrence.
Locke puts a limit on the naturalness of private property, however, by claiming that only enough must be taken to enjoy personally and to allow one's family to enjoy without spoiling and thus being of no use either to oneself or to others. Hoarding perishables, in other words, is unnatural in Locke's view. This does not mean, however, that it is unnatural to gather more than one can consume if the excess is gathered for the purpose of trade so as to obtain other goods that a person cannot either gather or produce on their own.
Since the naturalness of private property is given to it by labor, it is not enough to simply sit down in one place and claim that everything within certain boundaries from where one sits is one's private property. Some labor must be applied to the land in question before a person can claim it as one's own. It would seem from Locke's argument that a person also could lose the right to own this property if he or she applies no labor to it and thus does not make it useful either to oneself or to others.
Following the logic of Locke's argument, it would seem that a person could walk into a public park, measure out a lot of land, and begin applying labor to it - planting crops, building structures, cutting trees, etc. - which in turn would allow this person to call this plot of land private property. Locke counters this by claiming that human law and divine law intersect in this case. Specifically, god provides earth for humans to make the best possible use of it for their life and convenience, but humans themselves do not necessarily have to determine the way to use the land on the individual basis.
Often, humans unite to form a larger social body - village, town, county, or state - and decide how to use the land together. It is not uncommon for such bodies to come to a decision to leave some of the land in common possession. For an individual to attempt to make part of this common property into private property would reduce the usefulness of this land to others who own it together under "compact."
In this case, human law augments divine - or natural - law, and the actions of a person trying to take possession of such common property are judged on the basis of the human law. On the other hand, however, it would be unnatural and unlawful for any individual or group of individual to prevent someone from taking possession of land that has been previously unclaimed by anyone and has not been officially made common property by human law.
While private property is thus a natural right, can it also be considered a natural right to accumulate wealth? Locke argues in the affirmative by claiming that it is only unnatural to hoard perishables. Things that are durable and that possess certain value assigned to them by a given human community can be gathered through natural, honest labor beyond the limit placed on them by the individual's immediate needs.
Since they are gathered by labor, they represent natural private property and can be exchanged for other goods, both perishable and non-perishable, when the need arises. In addition, the accumulation of wealth is the pre-requisite for human progress, as only in a society where some excess wealth is generated can such wealth be spent on technological improvements and exploration - and subsequent use through labor - of new, previously unclaimed lands.