The study of the nervous system. Mostly stuff about the brain, hormones, nerves, and neurons.
If you want to know more about the brain and nervous system, study neuroscience.
Something which seems to have rendered the existence of a soul very, very unlikely, unfortunately.
Looks like this is the only life we're going to get folks.
Recent work on the brain has shown no evidence for souls, spirits, or any part of our personality or behavior distinct from the lump of jelly in our head.