Many people use the expression "private property" and even the word "mine" without being acquainted with John Locke's labor theory of property. They will find that theory stated with maximum clarity in Chapter V of his Second Treatise Concerning Civil Government.
Everyone, according to Locke, owns himself by right and the power invested in his mind and his hands. This is the simplest declaration of the fundamental injustice of chattel slavery, which is the ownership of one human being by another.
After this first step, Locke proceeds by distinguishing between the common and the proper. That which is not owned by anybody at all, yet maybe used by all, are certain tracts of public land that the community makes available to all members and can be used by them in common. The famous Boston Common is an example of this distinction. Forgetting the existence of Indian tribes in the New World of America, Locke refers to a time when all the world was like America, given by God to all human beings in common for their invasion and exploitation. What any human being, invading this God-given common, appropriated to himself or herself by his own labor became his or her private property.
When a man, for example, caught and tamed a wild horse, that became his horse; when he fenced in a plot of land and cultivated it by using his horse to pull tools that he himself made my his own labor, the resulting produce and the land itself became his private properties.
Locke thinks that no one should appropriate more than he can store for a future day. Nothing should be wasted. Enough should be left in the common for others to appropriate.
In one passage Locke considers the possibility of the solitary farmer employing another person to work for him. Let us consider this more complicated situation. Let Brown be the solitary farmer who, standing at his fence, accosts Smith, a passing journeyman, with a sack of clothes and other possessions slung over his back.
Brown says to Smith, "If you will work for me during the next week, using my land, my horse, and my tools, I will give you a share of what is produced during the week," Smith voluntarily accepts these terms.
That next week, Brown does no work at all. He stays inside his cabin, reading or thinking. At the end of the week Brown pays Smith off to Smith's satisfaction, and Smith goes on his way. Brown keeps for himself that share of the produce which is left after Smith has been paid off.
The question to be answered is: Did Brown rightfully earn that share of the week's produce which he claims for himself after he has paid Smith off, even though he himself did not do a stroke of work during the week that Smith labored for him?
Locke's answer to that question is affirmative. The reason is that even though the wealth acquired by Brown was not the result of his own labor, it could not have been produced by his servant if Brown had not put his own productive property to work during that week.
In other words, Brown is a capitalist who earns wealth without himself laboring but at the same time putting his capital (i.e., the means of production) to work.