Final answer:
Descartes' theory posits that the mind and body are distinct but interacting substances, with the mind as nonphysical and the body as physical. This view, known as dualism, suggests a separate existence of the mind (the 'thinking thing') from the material body.
Step-by-step explanation:
Descartes' theory of what the mind is argues that B. The mind and body are distinct substances that interact with each other. René Descartes, a French philosopher, presented this viewpoint in his theory of dualism. Dualism asserts that there are two fundamental and irreducible realities to fully explain the self: the mind (nonphysical) and the body (physical). Descartes refers to the mind as 'res cogitans' (a thinking thing) and the body as 'res extensa' (an extended nonthinking thing). He believed in the immortality of the soul, asserting that the soul exists prior to, and separate from, the body, and equated identity with the thinking self. This stands in contrast to materialism, which states that only material substance exists, and idealism, which posits that only spiritual substance exists. Descartes' famous dictum 'Cogito, ergo sum' (I think, therefore I am) emphasizes that thought can exist independently of physical form, although modern science has challenged this by showing that there is no evidence for thought without a brain.