The mind/body problem has been an issue in the philosophy of mind, dating back to at least Plato, who was the first dualist, and Aristotle, who was the first materialist. The mind/body problem is simple: What is the relationship between one's mind and one's body? Are they two separate entities, each able to survive on their own? Or as Descartes believed through his idea of interactionism, does the mind and body have a causal interaction?
How can a non-physical and physical substance causally interact? To answer this question, Descartes claimed that the pineal gland is the interface between the mind and the rest of the brain. He considered this to be the "seat of the soul". Physical states of the world influence our bodies, which influence our brains, which, through the pineal gland, influence our souls. Interactionism is the dualistic theory of the relation between mind and body, according to which physical events can cause mental events, and vice versa.
In this theory, the mind and the body are completely different entities. One can understand the body completely without adding any mental properties, and one can understand the mental properties completely without the body. The mind and body are totally unrelated. They interact through the pineal gland.
There are several reasons that this is not a satisfactory account of the mind and body relationship. Generally Descartes does not seem to have a clear view of what the "mind" is. Descartes says that he is unaware of anything but thinking that belongs to his mind. This proves that his mind could be something different from his body, and Descartes has no reason why body is not part of mind in some way that he's unaware of.
Materialism is the view that everything that actually exists is material, or physical. The only...