This is more like a houserule than a headcanon, but it mostly affects the setting: Thaumaturgy is the '0th' Circle of Sorcery. It can be used to perform Terrestrial Sorcerous Workings, e.g. summoning unbound 1st circle demons (using the rules for breaking the barriers). There are a fair number of thaumaturges in Creation; perhaps 1 in 1000 people. Occasionally someone with Thaumaturgy has the potential to initiate into Terrestrial Sorcery, so mortal Sorcerors seeking apprentices (such as the Sorceror-Princes of Ysyr) look first amongst village witches, tea-leaf readers and other such minor talents....