Our physical structure is hereditary. Yeah, true. What about our mental stuff? Our mind? The way the neurons connect between our brain cells to create thought?? That's hereditary too, isn't it?
To us, our deepest conscious self is our mind and our thoughts.
If this is not the real us, then what is the eternal 'soul' that we are, that has no birth or death?
How do we perceive our undying soul? Perhaps, is this 'spirit' referring to the life force? The force that keeps us breathing and our heart beating?