“The hard problem of consciousness is the problem of experience. Humans beings have subjective experience: there is something it is like to be them. We can say that a being is conscious in this sense – or is phenomenally conscious, as it is sometimes put—when there is something it is like to be that being. A mental state is conscious when there is something it is like to be in that state. Conscious states include states of perceptual experience, bodily sensation, mental imagery, emotional experience, occurrent thought, and more. There is something it is like to see a vivid green, to feel a sharp pain, to visualize the Eiffel tower, to feel a deep regret, and to think that one is late. Each of these states has a phenomenal character, with phenomenal properties (or qualia) characterizing what it is like to be in the state.

There is no question that experience is closely associated with physical processes in systems such as brains. It seems that physical processes give rise to experience, at least in the sense that producing a physical system (such as a brain) with the right physical properties inevitably yields corresponding states of experience. But how and why do physical processes give rise to experience? Why do not these processes take place 'in the dark,' without any accompanying states of experience? This is the central mystery of consciousness.

... it is conceivable that there be a system that is physically identical to a conscious being, but that lacks at least some of that being’s conscious states. Such a system might be a zombie: a system that is physically identical to a conscious being but that lacks consciousness entirely. It might also be an invert, with some of the original being’s experiences replaced by different experiences, or a partial zombie, with some experiences absent, or a combination thereof. These systems will look identical to a normal conscious being from the third-person perspective: in particular, their brain processes will be molecule-for-molecule identical with the original, and their behavior will be indistinguishable. But things will be different from the first-person point of view. What it is like to be an invert or a partial zombie will differ from what it is like to be the original being. And there is nothing it is like to be a zombie.

From the conceivability of zombies, proponents of the argument infer their possibility. ... zombies could have existed, perhaps in a very different sort of universe. For example, it is sometimes suggested that God could have created a zombie world, if he had so chosen. From here, it is inferred that consciousness must be nonphysical. If there is a ... possible universe that is physically identical to ours but that lacks consciousness, then consciousness must be a further, nonphysical component of our universe. If God could have created a zombie world, then ... after creating the physical processes in our world, he had to do more work to ensure that it contained consciousness."

“it seems every determination of movement happens from the impulsion of a thing moved, according to the manner in which it is pushed by that which moves it ... Contact is required for [this]. ... contact seems to me incompatible with the idea of an immaterial thing.”