It is difficult to truely own something in the ideal sense of the word. Most people pay a yearly fee to “own” their land. You cannot do whatever you want with your land i.e. you don’t really own it, but instead you own a lifetime tradeable lease. When you die, the land remains.

One definition of ownership I like is “can you freely destroy it without legal consequences?”.

Even “your” body: there are many laws about what you may and may not do with it (alive and dead).

There is even a grey area for whether you own your own thoughts? Maybe you rationally believe you can believe whatever you like, but actually your thoughts bleed through subconsciously into your actions which can have severe consequences. And unless you brought yourself up without human contact, it is very hard to discern the boundary between your own beliefs and those that belong to your society.