In the seventeenth century Descartes notoriously argued for the separation of body and mind, and in doing so he left his successors an intractable problem. On the one hand we have the teeming but essentially mechanical or physical world of mere meat: chemicals and currents, cells, nerves and blood. On the other hand we have the world of consciousness and spirit, thoughts, sensations, ideas, and selves. The problem is how to relate these two different realms. How does the world of the spirit interact with the physical world? How do thoughts and ideas make hearts pound, faces blush, fists clench, or legs race? The physical world seems quite self-sufficient. It takes a physical cause to have a physical effect, so surely there is no room for, and no need for, independent nudges from the mind? And conversely, how do physical events affect the mind? If I listen to some music the pressure of sound waves makes things happen in my ears, and excited neurons transmit currents into the teeming billions of other neurons inside my head. But then what? How does all that create the experience I have of listening to a Schubert melody or a Bach cantata? The whole thing seems unintelligible.

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