What is consciousness? What is this ineffable, subjective stuff—this thing, substance, process, energy, soul, whatever—that you experience as the sounds and sights of life, as pain or as pleasure, as anger or as the nagging feeling at the back of your head that maybe you’re not meant for this job after all. The question of the nature of consciousness is at the heart of the ancient mind-body problem. How does subjective consciousness relate to the objective universe, to matter and energy?