First we will consider the assigned baseball scenario under Leibniz's system of metaphysics. In the baseball scenario, the aggregate of the player, bat, pitch, swing and all the other substances in the universe are one and all contingent. There are other possible things, to be sure; but there are also other possible universes that could have existed but did not. The totality of contingent things, the bat, the player, etc., themselves do not explain themselves. Here Leibniz involves the principle of reason; "there can be found no fact that is true or existent, or any true proposition, without there being a sufficient reason for its being so and not otherwise." There must be, Leibniz insists, something outside the totality of contingent things (baseball games) which explains them, something which is itself necessary and therefore requires no explanation other than itself. .
This forms Leibniz's proof for the existence of God; a version of Aquinas's cosmological arguments. God, then, is the necessary being which constitutes the explanation of contingent being, why the universe is this way rather than any other. Not only is God the explanation of the baseball scenario but he is also the source of the intelligibility of such concepts as bat, swing and pitch. Leibniz goes further to prove the omniscience of God. If God is the explanation of the intelligibility of the universe, then God must have "access" to that intelligibility, such that God could be said to know what it is that being allowed to exist---that is, God must have the ability to grasp complete concepts. Not only does God constitute the contingent baseball game but he also knows what will take place before it happens. The pitch, swing and hit all take place not because God creates them but because he allows them. There is only one constraint on what God allows to happen, it must not violate Leibniz's other basic principle---non-contradiction.
