In overview, then, the divine ideas are those principles which account for the rationality and hence wisdom and non-arbitrariness of God’s creative and providential activity: when God acts, being good and wise he necessarily acts according to a preconceived plan, and these plans are the divine ideas. Implicit in this view, accordingly, is the notion that what is possible for God to do or make is determined by his ideas: God can do or make anything for which he has a divine idea, making the ideas the origin or source of divine possibility. Inasmuch as the ideas are located nowhere else than in the divine mind and essence itself, it follows that God’s own being is ultimately the origin and source of divine power and possibility. As Simo Knuuttila has put it, for Augustine the ideas provide an index or register of
all finite beings which could serve as partial imitations of the highest being. The ideas are divine thoughts and refer to possible actualization in the domain of mutability. They define the finite modes of imitating the infinite divine being. In this sense the possibilities have an ontological foundation in God’s essence… This was the dominating conception of modal metaphysics until Duns Scotus departed from it.
 Knuuttila, “Medieval Background,” 194. See also Knuuttila, “Time and Creation in Augustine,” 104 and Ross, “God, Creator of Kinds and Possibilities,” 320.