Rene Descartes believed in a theory of mind-body dualism. To him, there are two forms of matter, the mind, which is all tangible parts of the material world, and the mind, which encompasses the unseen, intangible, and unaffected by the body.
This naturally brought up the point that if mind and body cannot interact, how does the human mind interact with and control the human body? Descartes suggested that the human spirit was anchored to the body through the epiphysis/pineal gland.
Excersize: Pretend Rene Descartes' theory is entirely correct. What would happen if you had your pineal gland surgically removed? What this surgically remove your soul? What would happen if it suffered damage of some kind?
Advaita Vedanta suggests that dualism is the incorrect position, and puts forward a non-dualist view that the mind and the matter share one dimension, but furthermore, matter is actually illusory.
Excersize: Your senses are how you experience the world. What would the world be like if your senses were altered? How do you know what you are sensing is "real reality?" Consider how other people seem to perceive the world differently to you and how drastically different their realities may be.