Leibniz's philosophical system relied on the foundational principles of noncontradiction and sufficient reason. In his Discourse on Metaphysics (1686), Leibniz argued that the perfection of God was demonstrable through the principle of noncontradiction and that His perfection meant that God's creations must also be perfect. For various reasons, of course, the universe may appear to be far less than perfect but Leibniz invoked the principle of sufficient reason, which he held was basic to all reasoning, to demonstrate the perfection of creation. This principle states that there is no true fact or proposition without a sufficient reason for its being as it is and not otherwise. There is, in short, a reason for everything even though, in most cases, these reasons are not within the scope of human comprehension. If creation is perfect it is necessary to account for the existence of evil. Leibniz explained this partly through the principle of sufficient reason, and partly through the distinction between the activities of God and the activities of created things (an infinity of substances) that was based on the predicate-in-subject principle; a refinement of the principle of sufficient reason which held that a concept expressed in the predicate term of any true proposition is contained as well in the concept expressed by its subject term and this provides the grounds for the truth of the proposition. Leibniz defined substance on the basis of this principle and posited that a complete concept existed for each substance, which meant that all the predicates of the substance were intrinsic to it at any time--whether they had happened or would happen eventually. This raised serious problems with regard to contingency and free will since it might be argued that upon being created a substance (e.g., an individual human soul) already contained all the predicates (e.g. all the thoughts, actions, decisions) of that substance and that none of ...