  William did not compose the parts of the Magisterium in their rational order, and appears to have developed his conception of its structure as he wrote its various parts. He viewed it as having two principal parts. The first part is comprised of De Trinitate, De universo, and De anima. In this, the most philosophical part of the Magisterium, William proceeds by “the paths of proofs” or philosophizing rather than by an appeal to the authority of revelation. He intends in this part to combat a range of philosophical errors incompatible, as he sees it, with the Christian faith. In On the Trinity he develops his metaphysics, attacking, among others, errors regarding creation, divine freedom and the eternity of the world, and follows it with a philosophical treatment of the Trinity. William's vast work, On the Universe, is divided into two principal parts. The first part is concerned with the corporeal universe and is divided into three parts, the first arguing for a single first principle and for the oneness of the universe, the second taking up the question of the beginning of the universe and its future state, and the third treating the governance of the universe through God's providence. The second principal part is concerned with the spiritual universe, and is divided into three parts concerning, respectively, the intelligences or spiritual beings posited by Aristotle and his followers, the good and holy angels, and the bad and wicked angels, that is, demons. On the Soul is concerned with the human soul. It is divided into seven parts concerning, respectively, its existence, its essence, its composition, the number of souls in a human being, how the human soul comes into being, the state of the soul in the body, and the soul's relation to God, with a focus on the human intellect.