Final answer:
Rene Descartes believed in dualism, stating that the mind is non-material and separate from the physical body, an idea encapsulated in his statement 'Cogito, ergo sum' ('I think, therefore I am').
Step-by-step explanation:
French philosopher Rene Descartes posited the concept of dualism, which is the idea that the mind and body are distinct and separable entities.
He believed that the mind is non-material and capable of existing independently of the body. Descartes' famous statement Cogito, ergo sum ("I think, therefore I am") led him to conclude that because we think, we can be certain of our existence as minds, even if we doubt the existence of our bodies.
Therefore, according to Descartes, our minds are part of the non-physical realm, whereas our brains are physical objects within the physical realm.