 In speaking about the general features of the universe, ancient philosophers were inclined to think that all generic possibilities will be actualized, a habit of thinking called the principle of plenitude by Arthur O. Lovejoy (1936). Correspondingly, it was natural for them to think that the invariant structures of reality are necessary. This line of thought is found, e.g., in Plato’s doctrine of the ideas which are exhaustively imitated in the world by the Demiurge, in Aristotle’s theory of the priority of actuality over potentiality, in the Stoic doctrine of the rational world-order and the eternal cosmic cycles, as well as in Plotinus’s metaphysics of emanation (Knuuttila 1993).