 Until then, Everett was thought to be suffering from a problem of the ‘preferred  basis’:[19]  it was thought that without putting in by hand what should count as ‘worlds’, there were too many possible ways of defining such worlds, or too many ways of defining relative states. But looking for potentially relevant structures that are already present in the wave function allows one to identify worlds (or other relevant structures) without having to postulate the existence of some preferred states (whether or not they form an orthonormal basis). 