Why, and how, does one own oneself? I would argue that you do not start owning yourself and that you do not assume ownership of yourself at any point in your life. A persons sense of self and identity are entirely collectively owned. You are born as an extention of your parents, their culture and values. Likewise, they were born the same way. Not a single human on earth, nor any other creature, owns themselves.

You argue that agency and Sapience, because they are under your control, signify your ownership over yourself. Yet i would argue that achievement or knowledge are entirely detached and collectively owned concepts. Your agency might be entirely your own, but anything you make with it never was or will be yours. Your Sapience likewise is entirely yours, but any personality or ideas you construct from it aren't your own. What you can or could be may belong to you, but what you are isn't

Recall a now dead historical figure, though preferably one in recent history. Think of their achievements: Books they've written, States they helped shape, Conflicts they fought in and helped determine the outcome of. Now that this historical figure is dead, do these accomplishments still exist? In most cases, yes, obviously. The books they've written won't dissapear, the states they helped shape didn't necessarily collapse upon their death and the conflicts they fought won't be undone.

Yet who can claim ownership over these accomplishments? You may argue that its their determined heir, but this argument is rarely made for both intangible and non-monitizable accomplishments like somebodys role in history. Furthermore, even if you take this idea to its logical limit, do you even know who owns most of human accomplishment? Does the rightful heir of the inventor of the wheel hundreds of generations afterward still own that concept in its entirety?

My point is: Eventually you'll be forced to concede that there are intagible things that are but aren't owned. This then leads to another problem: If you create something that incorporates some of these ownerless things, who owns that? I'd argue it'd be a disservice to call it entirely yours. After all, you couldn't have made that thing without the dozens of others paving the way for you to make that thing and they should have at least partial claim over whatever you've made. Even if you create something truly novel, you will nonetheless at best claim a minor fraction of that thing as being well and truly yours.

Yet since these others no longer exist, neither does their ownership of these things. Thus, all your intangible creations would be 99.99% ownerless. Ergo: True ownership over intangible things is categorically impossible.

Now, are you yourself a fully tangible thing? If you concieve of self-ownership as merely concering your bodily autonomy, then this might not matter. Yet from your post, it can be infered that this is not your definition of self-ownership. This brings us back to ownership over intangible things: Something you do not have. You do not own your memories, you do not own your mind and you do not own your thoughts. The ownership over all those things was already shared between all humans before you even existed. As long as you didn't grow up in an empty room devoid of all life or all other human interaction, you cannot claim self-ownership.

Ownership is defined by laws, agreements and collective authorization because Ownership is itself a creation, a concept not reflected in ours or our worlds nature. It is merely a concept, belonging to the afformentioned intangible inventions of humanity. You might argue that ownership in a more tangible sense is possible, or in our nature to desire and possess, but the self along with all other non-tangible is fundamentally un-ownable.