If God does not have free choice to create this world rather than that world, then how do we explain contingency in the world? St. Thomas distinguishes between two types of necessity: absolute necessity, and necessity by supposition. For example, it is a necessity by supposition that Socrates sits, supposing that he sits. In other words, God created Socrates such that at this time and place he would sit, such that Socrates cannot stand when God creates him to sit. However, it is not absolutely necessary that Socrates sit, because absolutely speaking God could have created Socrates to stand rather than to sit.
But what happens if God has no choice what he creates rather than not creates? Then there is no distinction between absolute necessity and necessity by supposition. There is no supposing that God could have created Socrates any other way than sitting, and it is absolutely necessary that Socrates sit. But since contingency does exist in the world, God could choose what to create and what not to create.
I wonder how Plotinus managed to preserve contingency in his system. He subscribed to the principle of plenitude, meaning that the one emanates every possible species necessarily: since the One is also the Good, it cannot do anything other than what is best. Thus, this world is the best possible world, and it cannot be otherwise. How, then, does Plotinus account for contingency in the world?
Note that if God necessarily creates–even if God can choose which world to create–it becomes difficult to account for the differences between Plotinus’ One and the Christian God. Christians want to say that God exists separate from the world and is not dependent upon the world in any way. If God is not free to create rather than not create, then he cannot exist apart from the world. Perhaps we can even say that God somehow depends on the world, though I’m not quite sure how to explain that dependence yet.