It’s a police mantra that all members of the public are guilty of something, but some members of the public are more guilty than others. - Ben Aaronovitch

Policing is about moving from the unknown to the known and then further—to the provable.

Nightingale has always been reluctant to let me loose on the library and I must have frowned or something because he went on, “My worry with you, Peter, is not what you would learn but that, should you go into the library, you might never emerge again.”

Are you a practitioner?" I ask. "A wizard?"
"Sorceress, my dear," she says. "Practitioner is how the gentlemen have styled themselves, as quacks now style themselves as physicians.