Magic.

"This article deals with a real world topic. It may be integrated into canon to various degrees, or not at all."
Magic is a force that has not yet been confirmed or denied to exist in the world of the Shades of London. However, there is enough information in the books about the subject to warrant a page of its own, including speculation on how magic might work if it is indeed real, and how it is falsely preceived to work if not.
Whither magic actually exists is ambiguous. Paranormal methods to manipulate supernatural forces (such as ghosts) have never been explicitly shown or mentioned at all. The closest any characters has come to magical abilities first occurred in 1973, when a cult led by Sid and Sadie were poisoned in a ritual massacre while attempting to proform a magical rite. Jane Quaint was left by design as the only survivor in order to dispose of the bodies. 
The first sign of abnormal activity occured when Sid and Sadie, drank a mysterious concoction as part of a plan to stay young, and "fell asleep" for thirty years without showing any signs of aging or decay. 
Another unusual feature is that upon awakening, neither of them showed any signs of muscle atrophy, and in fact they were immediately able to get up and take effortlessly on three people in a fight. It would be difficult to explain any of this without the aid of magic, since Sid and Sadie were not shown to be connected to any sort of medical equipment in the interim. 
Performing rituals may be another requirement to use magic. Sid and Sadie base their rituals on Hellenism, specifically the Eleusinian Mysteries of Greece, so invoking deities such as Hecate and Demeter may be an important part of magic rituals in this universe. Other possible categories of magic are divination by using tarot cards, and technical necromancy.
Obviously, the word of two mass murderers should always be taken with a grain of salt, and most of Sid and Sadie's deeds could be given horrific but mundane explanations, such as cult brainwashing. 
The entire subject is confounded by the fact that subjects which could have been shown in a magical light, such as ghosts, the Sight]], and the terminus stones, have all been given fairly scientific explanations from the beginning, and it would be awfully late to introduce magical elements now.
Category:The Shadow Cabinet