Three points must strike anyone who has embarked on a study of dementia over a period of time. Firstly, that our conception of the syndrome is in a state of flux. Gone, for instance, in the past decade or two, is the requirement of a chronic, progressive, irreversible disorder for the diag­ nosis. I remember the surgeon who, when I was a student, returned a referral saying he would operate on the man when his dementia got better. Feeling superior, and encouraged by the consultant psychiatrist, we students laughed a good deal at this. Before we finished clerking on that Unit a visiting Professor of Psychiatry had demonstrated the reversibility of the symptoms of dementia in a patient with a rare metabolic disorder. Perhaps ignorance is sometimes an advance on received wisdom. The lesson is the concept of dementia must always reflect the state of knowledge and is therefore in a sense ad hoc. Secondly, what the criteria for, and also who the arbiters of, the diagnosis might be is not always clear. It is traditional to think that expressing opinions and making diagnosis of mental illness is almost a civic right, i.e.