This chapter poses the most difficult objection for the instrument doctrine, in particular as Aquinas conceives of it. For Aquinas, a created cause, Christ’s humanity, produces divine effects as an instrumental cause. But the tradition has affirmed that God alone is the cause of grace in the soul, and no created cause can produce grace. John Duns Scotus puts this objection to Aquinas’s account of instrumental causality, and this chapter argues that the criticism appears to succeed. If a created cause participates in the production of grace, as Aquinas argues, then Scotus argues that Aquinas fails to maintain the distinction of natures and powers in Christ basic to Chalcedonian Christology. For Christ’s humanity is taken up into God’s power and brings about the deification of the human person immediately, something only divine power can do. The ground is prepared for a response to this objection in the following chapter.