Since Galileo’s time, the physical sciences have leaped forward, explaining the workings of things from the tiniest quarks to the largest galaxy clusters. But explaining attributes that reside “only in consciousness”—the red of a sunset, say, or the bitter taste of a lemon—has proved far more difficult. Neuroscientists have identified a number of neural correlates of consciousness—brain states associated with specific mental states—but have not explained how matter forms minds in the first place. As philosopher Colin McGinn put it in a 1989 paper, “Somehow, we feel, the water of the physical brain is turned into the wine of consciousness.” Philosopher David Chalmers dubbed this issue the “hard problem” of consciousness.