A third significant question debated about between philosophers of the past is the mind-body problem. It is still wondered today whether the mind and the body are two separate entities, or whether the mind is part of the body. Ever since it was found that the brain is the part of the body responsible for thought processes, it has been considered that perhaps “the mind” is really just the brain. Then again, there may be a “mind” in addition to the brain that is responsible for some aspects of cognition (such as consciousness), and that does not die when the body does.

Descartes strongly believed that the mind and body were separate. His position, known as dualism, was based on his religious beliefs. Descartes asserted that when our bodies die, our minds (or souls) still exist. When it was called into question how this mind the interacted with the body, Descartes tried to make a diagram showing how visual sensations travel through the eyes to the brain, designating a certain step in the process as the part where the mind interacts with the body. However, even he was admittedly puzzled as to how a mechanical, material body could interact with a mind not made of material substance.

Descartes’ explanation was later criticized by Ryle, a philosopher of the twentieth century. Ryle pointed out that Descartes had viewed the body as belonging to the same “category” as the mind. Ryle believed that the mind was part of a different category, because its material and structure were different from that of the body.

Philosophers were grappling with the mind-body problem even before Descartes’ time. Democritus, an ancient Greek philosopher, developed the idea of materialism. He believed that everything in the world was made of tiny particles called atoms. Attributes of an object’s atoms, such as their size and shape, affect the form and properties of that object. Therefore, according to materialism, the human body and mind/soul are different because they are made of different types of atoms. A more modern variant of materialism is reductionism: the theory that all objects and events can be understood in terms of lawful behavior of the elements of which they are made. Reductionists state that therefore to understand the “mind” we need only understand the functions of the brain.

Leibniz had ideas along the same lines as the materialists. When this mathematician saw tiny living things under a microscope, he proposed that perhaps everything is made up of living elements, not unliving atoms. He called these living elements monads. Religion, in addition to science, played a part in Leibniz’s theory. He believed that God had ordered everything in the universe to be perfectly in harmony. Nothing influences anything else, although it may appear otherwise – God is the only influence. Therefore, monads that make up the mind do not actually interact with monads making up the body. They work in parallel, as part of God’s Divine Plan. This idea of Leibniz’s is known as psychophysical parallelism.