When you want to find your way around a city, you might memorise key streets or more likely use a simplified map as a guide as you travel. But when you know a city, you navigate by being able to recall how you get from A to B. In fact you may have difficulty drawing a map — certainly to scale — but your memory is made up of lots of instances of what lies around a particular corner. Much of what you learn about diseases is the map in this analogy. By contrast, what the experienced clinician knows are lots of instances of what lies round particular corners. Those instances have a name: they are called patients.