We cannot know how to fix a problem until we understand its causes. But even for some of the most common mental health problems, specialists argue over whether the answers lie in the person's biology, their psychology or their circumstances.

As a cognitive neuropsychiatrist, Anthony David brings together many fields of enquiry, from social and cognitive psychology to neurology. The key for each patient might be anything from a traumatic memory to a chemical imbalance, an unhealthy way of thinking or a hidden tumour.

Patrick believes he is dead. Jennifer's schizophrenia medication helped with her voices but did it cause Parkinson's? Emma is in a coma - or is she just refusing to respond?

Drawing from Professor David's career as a clinician and academic, these fascinating case studies reveal the unique complexity of the human mind, stretching the limits of our understanding.