 Early Waidan sources do not rely on the system of correlative cosmology. This system, instead, becomes essential in the later Waidan traditions and in virtually the whole of Neidan. The foundations of this new model of doctrine and practice were provided by the Cantong qi (Seal of the Unity of the Three), traditionally said to be written in the 2nd century CE but dating from between the mid-5th and the mid-7th century (Pregadio 2011). Scores of texts both within and outside the Daoist Canon are related to this work. Although its views have been understood mainly in relation to Neidan, they also apply to Waidan.