Incredibly controversial if you live in a country with a history of some form of feudalism or serfdom. Or one in which eminent domain laws exist. So basically, everywhere.

Property titles are legitimate if they are acquired through first-use i.e. homesteading (mixing your labour with an unowned resource) or via a contractual transfer from someone who has already acquired a title through homesteading.

For example,

If you step on an uninhabited island and declare that everything on it is yours, then that's not a valid title of ownership. Only that which you homestead can be yours.

And following this declaration, if you forcibly prevent someone from homesteading a previously un-homesteaded resource, that is unjust.

Practically, we live in a world where governments have 'declared' that they own everything and consider their authority to be the foremost in deciding what to do with unused resources.

The function of a legal system, state-based or market-based, ought to be to determine what is valid and just property. We don't live in that world. I consider those who want to bring about such a world to be hardcore advocates of freedom, utopian and idealistic as they may seem.