Gregory T. Doolan provides here the first detailed consideration of the divine ideas as causal principles. He examines Thomas Aquinas's philosophical doctrine of the divine ideas and convincingly argues that it is an essential element of his metaphysics. According to Thomas, the ideas in the mind of God are not only principles of his knowledge, but they are productive principles as well. In this role, God's ideas act as exemplars for things that he creates. As Doolan shows, this theory of exemplarism is an integral part of Thomas's account of the existence and order of the created universe....