Descartes stated that human possess a body which operates according to physical principle, and a mind that does not. He held that the mind and the body had a completely different nature. However Descartes showed that there is a particular relationship between the mind and the brain. They are 'dependent upon each other, only as a fountain pen and ink are interdependent. The pen will not write without the ink and the ink carries no message without the pen.' (MMM 3) The soul was was present in all the parts of the body, but the removal of any part of the body would not reduce the soul. For Descartes the body was procreated and the soul was created. He explained this further by stating that even if some parts of the body were removed, for example; a leg, the soul was still indivisible. Though the soul and the body were developed from two different natures, they could still react upon each other. Therefore, the body makes an influence on the body and the body makes an influence on the soul. This type of dualism that Descartes was subscribed to is called interactionism, which is also referred to as Cartesian dualism. He actually became the father of the mind-body theory of interactionism.
The mind was described as non physical as it was not located anywhere specific in the body. Descartes believed that if the mind was not closely related to the body, we could not have any feelings like hunger, thirst or pain. Descartes wanted to find the exact place where the mind influenced the body. He sought a structure in the brain as the brain stored the animal spirits and it was the only place which he believed that is not duplicate like all other brain structures. According to Descartes this structure had to be uniquely human as only humans own a mind. Descartes decided to choose the pineal gland as the point of interaction as it was surrounded by animal spirits. Still the soul was not something which should be considered as trapped in the pineal gland.