separated in thought and being from God. It was the function of the homoousion, the teaching that the Son is ‘of one being’ with the Father, to express the ontological relation between the Son and God the Father. While the precise meaning of this word in its historical context is the subject of much debate, the kind of function that it performs can be pointed quite simply. It is to establish a new ontological principle: that there can be a sharing in being. According to Greek ontology, to be is either