Patrick Grim has a well-known argument that there is no set of all true propositions. Suppose that argument is sound. For every set S of true propositions, there is a larger set S’ of true propositions. Since there is no set of all true propositions, it cannot be the case that anything knows every proposition in the set of all truths. Is this a problem for classical versions of omniscience? Here’s Alan Rhoda (see Enigman, too)
This creates a problem for omniscience if that notion is defined set-theoretically, e.g., believing every member of the set of all truths. One can argue along Cantorian lines that there can be no such thing as the set of all truths. That is, for every set of truths, one can construct new truths that are not already members of the set.
As Enigman points out, if this is right, then it follows that an essentially epistemically static (EES) God - i.e., a God who cannot either acquire or lose beliefs - cannot be omniscient (in a set-theoretical sense). Such a God cannot know all truths. Moreover, there would have to exist truths that are forever outside the ken of an EES God.
I confess to not quite seeing the problem described here, unless it turns on that specific set-theoretic definition of omniscience. Suppose we constuct a series of sets such that every set S contains only true propositions and is a subset of the succeeding set S’. We begin with what we would ordinarily call a saturated set of propostions (what Plantinga would call a maximal or complete and consistent set of propositions) and let each subsequent and larger set be constructed in the way Grim descibes. There will be infinitely many larger and larger sets and no largest set of true propositions.
Each set S in the sequence is consistent and contains propositions that hold at the actual world. If we define omniscience set-theoretically, could there be a classically omnisicient being in the actual world? Let an omniscient being be the following:
O. N(x)(x is omniscient iff. ~(ES)(S is a set of true propositions & x does not know every member of S).
(O) states that necessarily, for all x, x is omniscient just in case there is no set of true propositions such that x does not know every member of that set. As you move up in the sequence of sets of propositions, it is never the case that you reach some set S such that an omniscient being does not know every member of (that is, every proposition in) S. Since there is no set S such that an omniscient being does not know every member of S, an omniscient being knows every true proposition. That is perfectly possible even if the sequence is infinite. So there is no problem for classical omnisicence defined set theoretically, even if there is no set of all true propositions.
So, we arrive at two conclusions.
There is no set of all true propositions.
God knows every true proposition.
Addendum
O is not equivalent to O1, since the union of sets in S = the set of all sets of true propositions is not in S. We have conceded that the union of all sets in S is not a set, given Grim’s proof. But O1 entails that there is a set of all true propositions.
O1. N(x)(x is omniscient iff (Vp)(p is a true proposition only if x knows p)
This puts a somewhat interesting restriction on the logic of omniscience. There is one counterexample to the principle,
P. (Kp & Kq) only if K(p & q)
God will know each true proposition (and most conjunctions of true propostions), but he will not know the conjunction of all true propositions, since the conjunction of all true propositions (we should argue) is not a proposition at all.