Aquinas' interpretation of the metaphysics of the incarnation is an attempt to make sense out of a theological doctrine bequeathed to him as a traditional and central part of Christian belief. In this chapter, I want to explicate his interpretation of the doctrine and go some way towards defending it. It is not my intent to argue that the formulation of the doctrine he accepts as traditional is the only orthodox one, or that his interpretation of that formulation is the only appropriate interpretation of the doctrine, or that his interpretation is the best way to understand biblical statements about the nature of Christ. It is also not part of my purpose to show that Aquinas' interpretation is completely intelligible and coherent or philosophically defensible in every respect. Rather, my aim in this chapter is a limited one: to explicate Aquinas' interpretation of the doctrine of the incarnation in terms of his metaphysics in such a way as to clarify and support both his understanding of the doctrine and his metaphysics.
The formulation of the doctrine of the incarnation which Aquinas accepts and takes as binding on Christians is the one put forward at Chalcedon in ad 451: Christ is one hypostasis, one person, with two natures, one fully human and the other fully divine. Stating the Chalcedonian formulation is one thing; explaining what it means is another. Aquinas relies heavily on his general metaphysical theory to provide one interpretation of the Chalcedonian formulation. His interpretation is so thoroughly rooted in his general metaphysics that it is not possible to grasp this part of his philosophical theology without some understanding of his metaphysics. 1 On the other