Molinists urge that we can avoid necessitarian conclusion–the conclusion that there is just one possible world–if it is true in some worlds that God is not able to actualize the best world. This is false. The necessitarian conclusion follows from the plausible principle that God must actualize the best possible world, if there is a best possible world. I don’t think it’s difficult to show that there must be a best possible world, so I leave it as an exercise. Here’s the proof contra the Molinist.
1. Necessarily, God actualizes the best possible world. Basic Principle
2. God is essentially omnipotent, omniscient, omnibenevolent & necessarily existing (all as a matter of absolute necessity). Assumption.
3. w is the best possible world. Assumption
4. God actualized w in w. From 2,3
5. It is true in w that necessarily, God actualized w. From 1, 4.
6. Necessarily, God actualized w. From 5, S5
7. w is the only possible world. From 6
8. Necessitarianism is true. From 7.