Every generation needs an object of revolutionary fervour. For medicine at the end of the last century, it was the idea that all treatment should be founded on evidence. The revolutionaries demanded data behind each and every medical decision; the reactionaries argued that the void of data is precisely what a doctor’s expertise is supposed to fill. Those of us then only just entering the field simply wondered on what, if not evidence, medicine could have been based all this time.

As so often happens in the world of ideas, each side was wrong on a cardinal point they both agreed on. The error is not easy to see, so let’s proceed step-by-step.

The subject of medicine is the individual patient, its task to determine what to do in his or her specific case. Such judgment must naturally be drawn from the study of other patients, and so depends on how knowledge of individuals is distilled from knowledge of populations. This has traditionally been done with little formality, more often than not left as a matter of tacit experience.