From a medical perspective, consciousness is a description of our current level of awareness: people who are fully awake are completely conscious but, at the other extreme, people in a coma are without consciousness because they have no subjective thoughts or sense of awareness. Other states of consciousness, such as sleep and intoxication, sit between the two – awareness and subjective clarity are diminished but not completely absent.
From a philosophical perspective, consciousness is hard to define. Usually what is meant by the term is ‘phenomenological consciousness’ – a subjective feeling of what it is like to be that person or thing. Philosophers call these subjective conscious experiences ‘qualia’ (examples would be the redness of red and the bitterness of coffee). They are tricky for scientists to investigate, as we can never truly know if another person is having a subjective conscious experience.
Neuroscientists still don’t agree on how the human brain gives rise to a subjective sense of consciousness. One popular theory – the global neuronal workspace theory – likens the mind to a theatre, and proposes that when something becomes the focus of our attentional ‘spotlight’, this leads to a spread of neural activity beyond purely sensory processing areas, allowing the information or experience to reach the level of conscious experience.