A conscious mind in a sleeping brain: the title of this book provides a vivid image of the phenomenon of lucid dreaming, in which dreamers are consciously aware that they are dreaming while they seem to be soundly asleep. Lucid dreamers could be said to be awake to their inner worlds while they are asleep to the external world. Of the many questions that this singular phenomenon may raise, two are foremost: What is consciousness? And what is sleep? Although we cannot pro- vide complete answers to either question here, we can at least explain the sense in which we are using the two terms. We...