There has been much argument concerning the nature of consciousness and the mind. In 'The private life of the brain' Susan Greenfield avoids getting bogged down in such philosophising (in fact she is dismissive of non-physical ideas of the mind) and bases her ideas on what can actually be observed. She puts forward the idea of the 'neural correlate of consciousness', saying that different numbers of neurons being involved results in different states of consciousness. If you've ever wondered about the workings of your mind then you'll find that this hypothesis can explain a lot. The book requires no prior knowledge of the subject and is well worth reading.
Greenfield's ideas started with the question of why we take things so seriously when it would seem more fun just to 'live for the moment'. Her answer is that to live our lives to the full we need to do both. Thus as children we have fewer neurons operating together, and so we have a more spontaneous outlook on life, but as we grow older we try to make more sense of the world, which requires more neurons and a more serious viewpoint. Greenfield also examines abnormal states of mind - the use of drugs may reduce the size of the neural correlate, while if too many neurons become correlated then we enter a state of depression.