Retroactive Causality

In his brilliant, flawed These Three are One: The Practice of Trinitarian Theology (Challenges in Contemporary Theology) , David Cunningham notes how the doctrine of the Trinity implies retroactive causality: “At first, we might assume that a father precedes his son, both logically and temporally; but this is an illusion.  Certainly, the older man precedes the younger man; but the older man does not become a father until he has a child . . . . This aspect of divine relationality was emphasized in later Greek trinitarian theology, where the parental-filial language was thought to provide an especially clear description of two entities whose very existences were wholly dependent upon one another.  Similarly with respect to causal sequence: while there is a certain sense in which a parent is a cause of a child, there is an equally valid sense in which the child is the cause of the parent - for unless there is a child, the parent is not a parent!”

Cunningham argues that this is similar to the way Thomas goes about his Trinitarian theology, moving from the real relations of paternity, generation, spiration to the “subsistent relations” of the Persons.  We think persons must pre-exist their relations; Thomas says that in God’s life the relations are constitutive of persons.