It's as essential to human existence as water is to a fish. Every night we surrender it gratefully, only to get it back in the morning. We recognize that we have it, but we can never be sure anyone else does. Consciousness, this unique and perplexing mental state, has been the subject of debate for philosophers and scientists for millennia. And while it is widely agreed within contemporary philosophy that consciousness is a problem whose solutions are likely to determine the fate of any number of other problems, there is no settled position on the ultimate nature of consciousness.