  William is the first thinker in the Latin West to develop a systematic metaphysics based on the concepts of being (esse) and essence (essentia). Influenced by Boethius and Avicenna, he develops this metaphysics in his early work, On the Trinity, as a preliminary to his account of the Trinity, and he returns to it in other works, especially On the Universe. The central theme of his metaphysics is that all existing things, besides God, are composites of essence, on the one hand, and their being or existence, on the other, which is said to be acquired or partaken from God, the source of all being. God, in contrast, involves no composition of essence and being, but is his being.