The most important texts of the religion are those contained within the "Avesta", which includes as central the writings of Zoroaster known as the Gathas, poems within the Yasna that define the teachings of the Zoroaster, the main worship service of Zoroastrianism. The religious philosophy of Zoroaster divided the early Iranian gods of the Proto-Indo-Iranian tradition into ahuras and daevas, the latter of which were not considered worthy of worship. Zoroaster proclaimed that Ahura Mazda was the supreme creator, the creative and sustaining force of the universe through Asha, and that human beings are given a choice between supporting Ahura Mazda or not, making them responsible for their choices. Though Ahura Mazda has no equal contesting force, Angra Mainyu (destructive spirit/mentality), whose forces are born from Aka Manah (evil thought), is considered the main adversarial force of the religion, standing against Spenta Mainyu (creative spirit/mentality). Middle Persian literature developed Angra Mainyu further into Ahriman and advancing him to be the direct adversary to Ahura Mazda.