The Immutability-vs.-Omniscience Argument
1. If God exists, then he is immutable.
2. If God exists, then he omniscient.
3. An immutable being cannot know different things at different times.
4. To be omniscient, a being would need to know propositions about the past and future.
5. But what is past and what is future keep changing.
6. Thus, in order to know propositions about the past and future, a being would need to know different things at different times (from 5).
7. It follows that, to be omniscient, a being would need to know different things at different times (from 4 and 6).
8. Hence, it is impossible for an immutable being to be omniscient (from 3 and 7).
9. Therefore, it is impossible for God to exist (from 1, 2, and 8).
I think he went far too quickly on premise 3:
Premise 3 is beyond question because it simply draws out what's implied by the notion of immutability.
But premise 3 is not beyond question, because it's obviously ambiguous between two different readings:
3a. An immutable being cannot know things that are different at different times.
3b. An immutable being cannot know things in such a way as to know them differently at different times.
The difference it makes is considerable. 3a is much less plausible than 3b, and while many people who believe that God is immutable accept 3b, most people do not accept who believe that God is immutable do not accept 3a. If we accept 4, we are talking about knowing things that are at different times, given 5. Thus, 7 has to be understood as saying:
7a. To be omniscient, a being would need to know things that are different at different times.
Which, combined with 3a gives us the result that an immutable being cannot be omniscient; but combined with 3b gives us nothing, unless we sneak in a few hidden premises. If, on the other hand, we understand 7 as:
7b. To be omniscient, a being would need to know things in such a way as to know them differently at different times,
this does not follow from the premises on which it is supposed to be based, at least without supplementary principles.
So the argument is heavily ambiguous, and there is a lot to question about premise 3. And note that none of this requires rejecting 5; even if 5 is true, the ambiguity is a problem for 3 and 7. The problem, in fact, is a common one involving an illegitimate shift between knowing {x which has property p} and {knowing x} which has property p, i.e., between the modality of the object known and the modality of the knowing of it. For some reason people are especially inclined to do this with temporal modalities, since one runs into this illicit shift again and again in arguments against the claim that some things (e.g., God) are timeless.
