Perhaps you've heard Robert Nozick's famous "tomato-juice" argument. It's an argument against the Lockean labor-mixing idea. Basically, Nozick asks - if you can own something by mixing something you own with it, can I own the ocean if I dump a can of tomato juice in it? After posing this problem, Nozick puts forth his own theory of property - it is the creation of value that creates property.
Now, here's a thought. The Austrian economists (as well as general common sense) say that value is subjective. What does that make property, then?