One of the most difficult things for science to do is to measure the soul or spirit of people. The soul is something that is so extremely complicated that when we start to track it, we are not really sure of what we are tracking, or if we are tracking it alone and without interference from other things (like the mind), or if we are accurate with our extremely limited tracking abilities - psi factor.

Probably the soul, whatever it is, contains all the memories of the mind - the brain's memories - along with a whole pile of memories that the brain cannot receive, because the brain cannot sense all the things that the soul can.