The Source was defined as an object or place through which a ghost entered the world. The Source could be just about any object and therefore wasn't always easy to find. It sometimes held a special significance to the individual who had become a ghost in life, or it could be an object connected to the violence or trauma through which the individual had died.
Following the emergence of the Problem, anything that was considered to even have the possibility of being psychically charged as a "Source" was treated with fear and disdain, resulting in a rush to burn various old furniture and a multitude of antiques. Over time, however, a black market in such objects known as "relics" sprung up.
The prime duty of any agent when out on a case was to locate the Source and neutralize it. If done correctly, eliminating the Source would eliminate the ghost's ties to the Earth as well. A seal of silver or iron was often used on the Source to prevent the ghost's escape. Lucy Carlyle, however, discovered that sometimes by discovering the reason for the ghost's continued existence, however, it was sometimes possible to eliminate their presence without neutralizing the Source.