  Spinoza’s list of the basic types of existing things is  exceedingly short: substances and modes  (Ip4d).[2] Spinoza gives two extremely different, but equally striking accounts of how many tokens exist under each of these two basic types. There is only one existing substance, God, and there are infinitely many modes. Without yet unpacking these claims, we might ask Spinoza, “But could there have been more substances or fewer modes than there in fact are?” Or, keeping the number of tokens fixed, “Could there have been a different substance or different modes than the ones that in fact exist?” Surprisingly, Spinoza seems inclined to answer no to these questions. If so, then Spinoza thinks the actual world is the only possible world. This is the position of necessitarianism, a belief that few in the history of Western philosophy have explicitly embraced. And for good reason — on the face of it, necessaritianism is highly counterintuitive. Surely the world could have gone slightly differently than it has gone. Couldn’t the Allies have lost WWII? Couldn’t a leaf have landed an eighth of a centimeter farther to the right than it in fact landed? Similarly, surely the world could have contained different individuals in different numbers than it in fact contains. Couldn’t Leibniz have had a sister or not been born at all?