How do we know for sure if someone is unconscious? After suffering a stroke, Kate woke up from a coma able to see, hear and feel, but unable to move a muscle. For two weeks doctors and her family and friends thought she was in a vegetative state. Andrew frequently sleepwalks around his house and argues convincingly with his partner that he is awake. When he wakes up he has no memory of their conversations. Following an operation, Rosie’s 2-year-old son came out of anaesthesia like “devil spawn”, thrashing about and unable to recognise her. Anaesthetists call this “emergence delirium” and don’t fully understand how it happens. There have been significant gains in our scientific understanding of consciousness, but exactly how it works – and what it is – remains a mystery. This week, Insight looks at what we do know about consciousness and asks whether we can know for certain if someone is conscious or not.