We don’t even understand the most basic, important aspect of our existence: Who am I? Stephen Levine, in his excellent book "Who Dies?" (Ref.1), stipulates that we almost always perceive "me" as "my body," yet there are mind games which show us that it is unclear where the boundaries of "me" actually are. For example, try to determine exactly where/when a visual image arrives at "me." It's not at the eyeball or the optic nerve, or probably even the occipital lobe. Similarly, imagine replacing every cell of your body with an identical, but different, cell. Would you still be "you?"
References:
1. "Who Dies" by Steven Levine