What forms can consciousness take and can we see it in our brains?
In essence, consciousness is any kind of subjective experience. Being in pain; smelling onions frying; feeling humiliated; recognising a friend in the crowd; reflecting that you are wiser than you were last year  all of these are examples of conscious experiences. In a field fraught with disagreements, this is something that most, but not all, researchers agree on. Go any deeper, though, and the rifts open up. The 17th-century French philosopher Ren Descartes famously divided the universe into matter stuff, such as rocks and physical bodies, and mind stuff. In the 20th century, philosopher David Chalmers at New York University built on Descartess separation, known as dualism, and the work of later thinkers, to distinguish between easy problems of consciousness and the hard problem. The easy stuff consists of explaining the brain processes associated with consciousness, such as the integration of sensory information, learning, thinking and being awake or asleep. Though we are making steady progress, these problems have yet to be cracked: they are easy only in the sense that the known strategies of cognitive and neuroscientific research should eventually provide full explanations. The hard problem, which Chalmers introduced at a scientific meeting in 1994, is to explain why and how we have subjective experiences at all. Consciousness poses the most baffling problem in the science of the mind, Chalmers said. When we think and perceive, there is a whir of information-processing in the brain, as he put it, but also very distinctive subjective states of mind. The puzzle is how a 1.3 kilogram organ with the consistency of tofu can generate the feeling of being. 