If we spread credence equally over all possibilities and have reason to think that cases where inductive inference is unreliable require a very specific arrangement of things, those unreliable cases form a small fraction of the total space of possibilities.

Premise P3 can perhaps be challenged on the grounds that a priori justifications can also be given for contingent propositions. Even though an inductive inference can fail in some possible situations, it could still be reasonable to form an expectation of reliability if we spread our credence equally over all the possibilities and have reason to think (or at least no reason to doubt) that the cases where inductive inference is unreliable require a ‘very specific arrangement of things’ and thus