What is consciousness? What does it do? Could we have evolved without it? ‘Why the mystery?’ considers the definition of consciousness and how psychologists, neuroscientists, and philosophers have tried to explain it. From the Cartesian dualism of René Descartes to the ‘hard problem’ of consciousness, a phrase coined in 1994 by the Australian philosopher David Chalmers, and the question ‘What is it like to be a bat?’ of American philosopher Thomas Nagel, it is shown that there is no generally agreed definition of consciousness. Subjectivity (or phenomenality), qualia, and the ideas of philosopher Daniel Dennett are also discussed.