What would persuade me to believe that such ownership is invalid?
First, we need to discard the entire idea of ownership; if I improve, tend, and live on a piece of ground that has been in the habit of thinking of me as its owner, is that enough? OK ground doesn’t think, so next question – who does think? And when they think about the ground I care for and live on, exactly what might they think? Since the idea of ownership is moot, we can venture into anarchy in which, since no one owns anything, anyone can take and use anything since ownership doesn’t exist.
That doesn’t sound practical. Do we get to look at this as a moral issue, or merely a practical one? I ask this because the question of whether nor not ownership is a valid concept usually arises in a moral context not a practical one.
But if we stipulate that some variety of morality is in play, who can provide a viable definition?
IMHO the ownership question must sit on the table until that larger issue finds a definition satisfactory to all. I’m prepared for a significant delay in reaching that point.
Meanwhile, questions of morality etc. which arrive naked of even a working definition will have to defer to the practical side. Systems which deny the validity of individual ownership tend to be chaotic, since in the end “the people” own everything, and “the people’s designated representatives” discover that they live on Animal Farm (George Orwell).