The latest (July 2011) Faith and Philosophy contains an excellent article by Jeff Speaks on some difficulties related to establishing the consistency of certain claims (he uses as examples the existence of human freedom and the existence of evil) with the existence of an Anselmian God. The basic idea is this: since an Anselmian God is, by definition, a necessary being, establishing the possibility of an Anselmian God is tantamount to establishing the necessary, and therefore actual, existence of an Anselmian God. But these compatibility arguments typically, in one way or another, assume the possibility, and so the actuality, of an Anselmian God. If we were allowed to assume this premise, our task would be extremely easy! We could argue as follows:
- God (actually) exists
- Evil (actually) exists
- The existence of God is consistent with the existence of evil.
Piece of cake! Now I, of course, take this argument to be sound. In fact, I even think that some people (depending on their background beliefs) might be rational in allowing this argument to increase their confidence in the truth of (3). But clearly this argument cannot be used to respond to atheist arguments from evil against the existence of God. It is dialectically inadmissible in that context.
In his paper, Speaks argues that Warfield's argument for the compatibility of necessary omniscience with human freedom and Plantinga's free will defense are both a lot like this. That is, they both assume that, possibly, an Anselmian God exists. But if that assumption is admissible, then we could just use this simpler argument. But obviously we can't use this simpler argument, so the premise must be inadmissible. (This isn't exactly the way Speaks puts his points together; it's my interpretation of what his arguments actually show.)
Speaks states the "principal conclusion" of his paper as follows:
any argument for the compatibility of two propositions must also be an argument for the possibility of each of those propositions. Hence it is impossible to argue for the compatibility of two propositions, one of which is necessary if possible, without arguing for the truth of that proposition. (p. 291)
In this post, I'm going to push back.