Human beings are material objects. However, unlike other material objects (e.g. non-living things) humans have the ability to form judgements and reason their existence. Meaning to say that, human beings have 'minds'. In general, humans are characterised as having both a mind and body. By definition, mind is referred to the mental processes, thought and consciousness whereas body is referred to the physical aspects of the brain-neurons and how the brain is structured. This is known as dualism.
In the philosophy of mind, dualism is the theory that the mental (mind) and the physical (body) are both real or exist, but both of them are different kinds of thing. The theory of mind-body dualism is presented by Descartes, who holds that both mind and body are substances, in which the body is a material substance as it is extended in space whereas the mind is not extended in space, and so called a metaphysical substance. According to Descartes, he believed that mind and body actually can interact through the pineal gland in the brain. In Descartes’s first principle of philosophy, “I think, therefore I am”, makes mind more certain than matter. It also showed that the mind which is a thinking thing can exist apart from its extended body. Hence, Descartes said that the mind is a substance that is different from the body (a substance whose essence is thought). This became known as “Substance Dualism” (view that the mind and body function separately, without interchange) or “Cartesian Dualism” (view that there is a two-way interaction between mental and physical substances).
Substance dualism asserts that mind and matter are completely two different kinds of substances, in which it claims that immortal souls occupy an independent “realm” of existence distinct from that of the physical world. So from here, the mind is not just a collection of thoughts, but it is that which thinks, an immaterial substance over and above its immaterial states.