Why we dream: the definitive answer tells the remarkable story of how Joe Griffin discovered how and why dreaming evolved in mammals and helped us unravel what our dreams actually mean. Thanks to Griffin's work we now know what dreams are doing for us: they keep us sane, or, in certain circumstances, can drive us mad (psychotic). And this knowledge opens up wonderful new possibilities for humanity: greater creativity; improved mental health and deeper understanding of who we are. Griffin and Tyrrell convincingly show that dreaming is vital for mental health and that the brain state we associate with dreaming (the REM state) also has crucial importance for when we are awake. This understanding of the REM state explains not only how our brains construct a model of reality, but also explains hypnosis, how creative behaviour works, and why we develop mental illnesses such as depression and psychosis.