Natural law says that natural rights come from the ability of humans to reason.
Reason tells an individual mature person that he has an "a priori" human action. It also tells that person that other people "can" also have desires an execute purposeful actions (empathy).
It eventually reveals to that person that others may pretty probably dislike being robbed (of certain things...), killed, enslaved, etc because he wouldn't like that done in himself
"Private property" is just an idea inside the human mind. A useful idea, though 🤷