Dr. William Lane Craig has made a response to my previous post where I argued that his own view of divine sovereignty entails the truth of divine simplicity. Now, Craig is actually correct about one thing: My argument does not by itself entail that God is identical to all his parts. This only follows from the conclusion of my argument if you grant that God really has a will, intellect, etc. Craig does not grant this, since he doesn't think talk about things having parts is metaphysically substantive.
There are a lot of things to say about Craig's response here. Maybe the first is to simply note that he is denying that anything really, in the metaphysically deepest sense, has any parts. This is surely an unacceptable conclusion. Personally I would think it's better to simply deny God has any parts rather than to deny anything has parts. Absent this option, if I didn't believe in divine simplicity I would even modify my account of divine sovereignty just to save parthood. For otherwise I honestly don't know how Craig explains kidneys, brains, legs and their relations to the people who have them. This is just a datum of experience, that there are at least some parts.
Craig tries to use an argument by Peter van Inwagen to back up his thesis. However, the problem is that Van Inwagen's argument only demonstrates the falsity of the doctrine of arbitrary undetached parts, which is the idea that any region of a body can be taken to be a proper part. His argument can only go through if we are dealing with 'parts' like Dottie* which are constituted by enough matter in such a form that a person can survive by becoming identical to them. It's not obvious though that I could ever become identical to, say, my heart. So his argument would not go through with those sorts of proper parts.
Now, I'm inclined to reject the doctrine of arbitrary undetached parts anyway so I'm happy to accept the soundness of the argument. But it just doesn't demonstrate that there are no proper parts. And if it did entail that, then--like Peter Geach did with Tibbles the Cat--I would just take the argument to establish the relativity of identity rather than the complete lack of proper parthood. More importantly, it's not even obviously sound. We might just deny the premise that Dottie becomes identical to Dottie*, since Dottie seems to be an animal (or a soul) and Dottie* seems to be a 'lump'. In virtue of their falling under different sortals these two objects have different identity conditions associated with them, and thus by Leibniz's law they are non-identical. They are merely constituted by the same matter.
There's also something to be said about Craig's underlying Carnapian sympathies. There is intense debate about taking this sort of view about language and metaphysical methodology (cf. the Chalmers volume on metametaphysics), and suffice it to say for now that I'm not too sympathetic. I will criticize this neo-Carnapian line of thought later, but this post should be enough to see why Craig's response is inadequate.