There are different types and historical layers of Abhidharma literature. The early canonical Abhidharma works (like the "Abhidhamma Pitaka") are not philosophical treatises, but mainly summaries and expositions of early doctrinal lists with their accompanying explanations. These texts developed out of early Buddhist lists or matrices ("mātṛkās") of key teachings, such as the 37 factors leading to Awakening. Scholars like Erich Frauwallner have argued that there is an "ancient core" of early pre-sectarian material in the earliest Abhidharma works, such as in the Theravada "Vibhanga", the "Dharmaskandha" of the Sarvastivada, and the "Śāriputrābhidharma" of the Dharmaguptaka school.