According to a strong and unrestricted version of the principle of sufficient reason, everything that exists or occurs has a sufficient reason for its existence or occurrence, where sufficient reasons are taken to entail or necessitate the existence or occurrence of the entity in question. For our purposes, call this version PSR.
Peter van Inwagen famously argued that PSR is implausible, as it entails that everything exists or occurs of metaphysical necessity, and that this is absurd (since many things seem to be contingent and not necessary). Call this the PvI objection. Now people tend to react to the PvI objection in one of three ways:
(i) Hold on to PSR, while accepting that it entails everything exists or occurs of metaphysical necessity.
(ii) Restrict PSR so as to make conceptual space for things to occur contingently.
(iii) Reject PSR in order to make conceptual space for things to occur contingently.
However, it seems to me that there is another option:
(iv) Hold on to PSR and its implication that everything exists or occurs of metaphysical necessity, but deny that it entails that everything exists in our universe exists or occurs of necessity in all universes.
To motivate this option, note that the PvI objection gets most of its bite by assuming that
1. Our universe is the only universe that exists at the actual world.For then it seems to follow from (1) and the PvI objection that
2. Whatever exists or occurs in our universe at the actual world exists or occurs in all possible universes at all possible worlds.However, if it's possible that there is a multiverse comprising an infinite number of universes (where these are sufficiently diverse so as to capture our modal intuitions about the space of metaphysical possibilities), then this inference doesn't go through.
Option (iv) exploits the epistemic possibility of a multiverse of the sort sketched above to block the inference from (1) and the PvI objection to (2). Those who take this option grant that PSR entails that everything exists or occurs of metaphysical necessity, in the sense that the multiverse at the actual world exists at all possible worlds. However, they deny that all that exists or occurs in our tiny corner of the multiverse exists or occurs at all other universes within the multiverse. In this way, they deflate the epistemic significance of the intuition of contingency that gives the PvI objection its bite. By choosing option (iv), then, one can have one's explanatory cake and eat it, too.
I'm not saying that option (iv) doesn't raise questions of its own. (E.g., does it commit one to Lewisian modal realism and/or counterpart theory? Etc., etc.). All I'm saying is that (iv) is an option that's no worse off than the others, and that it's not clear that it fares worse than PvI's option in terms of a theoretical cost-benefit analysis between the respective views. At a minimum, it's an option that deserves more consideration than it has hitherto received.
Allison Krile Thornton reviews the book for NDPR .
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