The answer, I think, is that one can just see upon reflection that these things are impossible.... One can see that a physical object just can't do that sort of thing [i.e., think]. This isn't as clear, perhaps, as that a proposition can't be red; some impossibilities are more clearly impossible than others. But one can see it at least to some degree. And the same doesn't go for an immaterial thing's thinking; we certainly can't see that no immaterial thing can think (57f).
I don't buy it and here's the quick argument I've run over there:
(1) In general, inspecting the material parts does not put one in a position to see what could arise once the material parts are in place.
(2) As such, only people with special epistemic powers could see what Plantinga claims to see.
I think we get some support for (1) if we think about normative properties. If you cannot derive an 'ought' from an 'is', it seems that inspecting the 'is' properties will not put us in a position to appreciate what 'ought' to be. You can restrict (1) so as to exclude normative properties, but the point seems perfectly general and I can't think of any reason to accept (1) on its restricted reading. I suppose there's a question as to how to link (1) and (2). The implicit assumption is that if we ought to expect that we could not see that p is true on the hypothesis that it is, we ought to be sceptical of anyone's claims that they can just see that p is false. So, I guess we can infer (2) from (1) and demand more from those who reject materialism.