John Gunn, emeritus professor of forensic psychiatry at the Institute of Psychiatry, used to tell the Maudsley registrars that a copy of the Mental Health Act should be to them as a stethoscope is to a physician – they should always keep a copy to hand. The latest edition of this small book, published jointly by the British Medical Association and The Law Society, also lays claim to must-have status – not simply for psychiatrists, but all clinicians, because (as the authors indicate on p. 31) courts tend to regard any doctor as having expertise in assessing competence. Since it is self-evident that the majority of doctors do not in fact possess such expertise, those who are required to give an opinion on mental capacity need this book much more than they need a stethoscope.

The coverage of the book is comprehensive, including (for instance) competence to consent to medical research. The style is succinct and completely clear – a pleasurable rarity from lawyers. Its counsel is wise: for example, in section 3.5 (pp. 34–5) professionals are advised against witnessing documents without first competently assessing mental capacity and recording a contemporaneous opinion.

The sands of time are flowing away from this book, because a new mental capacity act is currently being forged in Parliament and it seems inconceivable that a third edition is not being planned to coincide with the implementation of the new act, expected in 2007. Meanwhile, if this is not Gunn's stethoscope, then it should at least be designated his patella hammer.