The philosopher who believed that the mind and body are mentally independent in their function, even if they exist physically together as a whole, is René Descartes. He is closely associated with mind–body dualism, which holds that the mind is a nonphysical substance distinct from the physical body. Descartes argued that the mind (a thinking, non-extended thing) and the body (an extended, non-thinking thing) are fundamentally different and can exist independently of each other. This view is known as substance dualism and implies that the mind and body are separate substances that interact but are not the same entity

. Descartes famously formulated the mind–body problem in its modern form and maintained that the mind is the seat of consciousness and self-awareness, distinct from the brain as a physical organ. He also discussed how the mind and body are united to form a human being, despite their distinct natures

. In summary, René Descartes is the key philosopher who believed in the mental independence of mind and body while acknowledging their physical coexistence as a whole.