No one denies the reality of infection, heart disease, pain or cancer – or questions the scientific rationale behind the design of drugs to relieve or cure these conditions. With mental illnesses, such as depression and schizophrenia, the situation is a little different. The development and use of pharmaceutical drugs which work on the mind is one of the most controversial areas in medicine today (recreational drugs work on the mind too, and are also the subject of controversy. We will look at these in the next chapter).

If you flip through the pages of the Diagnostic and Statistical Manual IV (commonly known as DSM IV) you might think that psychiatry is as cut and dried a scientific discipline as any other branch of clinical medicine. DSM IV is the American Association of Psychiatry's classification of mental illness and on first glance it looks as authoritative and impressive as the Oxford Textbook of Medicine. A closer look reveals some curious – and fascinating – mental diseases. What about ‘oppositional defiant disorder’ and ‘bereavement reaction’? Here the normal spectrum of human behaviour (childhood disobedience and grief, respectively) appear to undergo a subtle shift into psychological pathology. Classification of physical disease is usually more clear-cut.

For most of the mental conditions in DSM IV there is no physical diagnosis such as a blood test or scan, which also sets them aside from most physical diseases. An exception is Alzheimer's disease, which – as we shall see – is associated with specific brain lesions, currently only detectable post mortem. Instead of running lab tests, psychiatrists and family doctors rely on their own observations and questioning the patient.