Guise theorists think that "London is beautiful" and "Londres est belle" express the same proposition, but under two different guises. That's what makes it possible for Pierre to affirm the first statement (he does so under the "London" guise) while denying the other (under the "Londres" guise), without being irrational, if he doesn't know that London is Londres.
A standard definition of omniscience is:
- An omniscient being is one that knows every truth and believes only truths.
- An omniscient being is one that knows all truths under all their guises and believes only truths.
- An omniscient being is one that knows every truth under some guise or other and believes only truths.
If we take (2), however, then we get an interesting result: If there is an omniscient being, there are no creaturely essentially private guises. An essentially private guise would be one that only one individual can believe something under. A creaturely one would be one where that individual is a creature. For if there were any creaturely essentially private guises, there would surely be some truths under them, and so by (2) God would know these truths under these guises, contrary to the privacy of the guises.
The most plausible kind of private guise would be "I". One might think that if I believe that I now typing, while you can believe the same proposition that I believe, you cannot believe it under the same guise. To believe it under the guise corresponding to "He is now typing" or "Alex is now typing" is to believe it under a different guise. Otherwise guise theory can't solve the Pierre problem. For it is possible for me to believe that I am now typing without believing "He (Alex) is now typing" or "Alex is now typing". If (2) is true and there is an omniscient being, then "I" is not an essentially private guise. That is surprising.
Moreover, one might think that there are guises which are essentially tied to certain experiences. Suppose I am suffering an odd pain. I believe the necessary truth "This is a pain" where the "This" refers to the pain under a guise whose grasp by me depends on my actually having or remembering a pain like that. If a being exists that is essentially omniscient, doesn't necessarily suffer and essentially lacks false memories, then such a being would have to be able to grasp "This is a pain" propositions under such pain-guises without either having the pains or remembering them.
So adopting (2) leads to surprising consequences for guise theory. Given the plausibility of the possibility of an omniscient being, this is a problem for guise theory.
Is there an alternative to (2) and (3)? Maybe one could say that an omniscient being knows truths under some divine guises which are richer than the guises under which we know things. Thus, perhaps God doesn't know that I am typing and that this is a pain under the guise under which I know it, but he knows them under a richer guise. The challenge here would be to try to work out what the greater richness of guise could be, without it being some kind of inclusion of the more private-seeming guises.
Of course, one could adopt (2) and simply accept the surprising consequences. Surprising consequences may be some evidence against a theory, but they can be far from refuting it.
Nor will abandoning guise theory get out of these problems. Guises were introduced to solve certain problems, like how it is possible to rationally believe "I am now typing" without believing "Alex is now typing" (say, I've forgotten my name). These problems still need a solution. A Fregean solution will say that all these sentences express different propositions. Fine. But the intuitions that supported privacy of some guises will now support the privacy of the corresponding propositions.
So, the existence of an omniscient being shows that some things that seemed to be private aren't.