I think the arguments of Monism vs. Dualism are similar to those of Science vs. Religion. They will never end, and yet they share large borders with each other. For example, a Materialistic Monist would say that "Mind" is a concept which refers to an individual’s subjective and conscious, experience through the body’s 5 senses, and it resides inside the brain. Brain is a part of body, and body is physical, therefore the mind is physical, and the physical world is the only existing reality. A Dualist would argue that mind has its own place other than body, so when the physical body dies and doesn’t exist anymore, the mind would still exist and it would fly away like spirit. Again the Materialistic Monist would dispute that People are made of atoms, and atoms are entirely physical objects, with nothing but physical properties and physical relations to one another. At this point the Dualist would respond that maybe atoms do have some undiscovered properties other the physical mass, charge and position. People experience pains, emotions, and desires. Therefore they have mental states. The materialist would again defend his ideas by using the atomic and evolutionary theories, and say that "Mind" is a concept which refers to an individual’s subjective and conscious, experience through the body’s 5 senses, coming from brain which is physical. This argument would continue forever.
Now imagine if there were two different realities, mental and physical. How do these two worlds interact with each other and affect each other? The answer to that question depends on what you believe in. Cartesian dualists who follow Descartes and are religious believe that God has given us the innate power of having our minds and bodies communicate with each other in a contactless manner. Other dualists have turned to different beliefs. Some believe in Interactionism which follows common sense by giving the view that there’s a two-way causation between the mind and the body. However, Interactionism only follows common sense and that isn’t enough to prove anything for some dualists. These people have endorsed Epiphenomenalism, which says that there’s only a one-way causation between the mind and the body, and mental causation doesn’t exist. Thus, only physical events give rise to mental events, but not vice versa. Epiphenomenalism may avoid some of the problems of dualism, but some problems still remain unanswered. Some other dualists resort to Parallelism and avoid all the problems. Parallelism denies that the mind and the body interact at all. According to parallelists there is no forthright causal link between the mind and the body.
Our minds and bodies keep interacting with each other all the time. The mind is giving the body messages to function all the time. Scientifically, these messages are in fact electrical currents moving from the mind throughout the body, inside the nervous system, in milliseconds. The body cannot function without the mind. This is why those who are brain dead cannot live, move nor interact with others anymore. The functionality of the body is dependent on the mind. Descartes believed that the mind could exist without the body. On the other hand, however, the mind is part of the brain, and brain is a physical entity that requires oxygen and food so cannot exist without the body. The mind and the body are dependent on each other, and if one is not working properly it can affect the other’s functioning.
The Mind-Body problem is one of the oldest philosophical problems. Plato was among the first dualists and Aristotle was one of the first materialists in history. Nowadays many modern philosophers have rejected the idea of mind and matter existing as different substances. However, many are still realists about mind. There are many different beliefs, intuitions and ideas about how mind and body interact with each other, but since it’s still difficult to draw a strict line between the mind and the brain, it’s still not easy to give a certain answer to that question. Many people think that it’s safe to say that mind and body are related and their functionality and existence depends on each other. Nonetheless, the argument of Thomistic Monism vs. Cartesian Dualism still continues.