Mind-body Dualists believe there are two different realms that define us. One is the physical realm, well studied and understood by the laws of physics, while the other one is the non-physical realm, where our selves exist. Our essence, our soul, if you want, exists in this non-physical realm, and it interacts and controls our physical body through some as yet unexplained mechanism. Most religions are based on a dualist theory, including Christianity, Islam, and Hinduism.
On the other side of the discussion are Monists, who do not believe in the existence of dual realities. The term monism is used to designate the position that everything is either mental (idealism) or that everything is physical (materialism).
Raymond Smullyan, deceased two days ago (February 10th, 2017),
had a clear view on dualism, which he makes clear in this history, published in his book This book needs no title.
An Unfortunate Dualist
Once upon a time there was a dualist. He believed that mind and matter are separate substances. Just how they interacted he did not pretend to know-this was one of the “mysteries” of life. But he was sure they were quite separate substances.
This dualist, unfortunately, led an unbearably painful life-not because of his philosophical beliefs, but for quite different reasons. And he had excellent empirical evidence that no respite was in sight for the rest of his life. He longed for nothing more than to die. But he was deterred from suicide by such reasons as: (1) he did not want to hurt other people by his death; (2) he was afraid suicide might be morally wrong; (3) he was afraid there might be an afterlife, and he did not want to risk the possibility of eternal punishment. So our poor dualist was quite desperate.