The very concept of an immaterial substance is unintelligible. Attempts to make them intelligible render them into ordinary physical substances at the next level up, so to speak. And it is of course out of the question to simply say that an immaterial substance is perfectly intelligible since they are just minds (as Plantinga seems to do). It is obvious that there is thinking but it is not at all obvious that an immaterial substance could think. What would that even mean? The upshot then is that substance dualism is not a viable theory.
I think this runs into problems when one considers what actually makes a theory viable rather than glossing it over with this extremely ambiguous question, "What would that even mean?" For it is an extremely ambiguous question. There are several different questions that could be asked in those words, and the answers to each of those questions do not impact on the question of the viability of a theory in the same way. The spread of possibilities when we are asking about the intelligibility of something is closely related.
Imagine someone trying to figure out what scientists mean when they talk about black holes. "What would that even mean?" or "What makes that intelligible?" (asked in good faith) may mean, "What reasons could one have for believing that this is true?" In that case, the physicist would reply by giving his or her reasons to think that there are, in fact, black holes -- certain astronomical phenomena and theoretical considerations based on general relativity, for instance. More broadly, it might mean, "What reasons could one have for believing that this is possible?" The same type of evidence would suffice for answering the question; we are just dealing with a weaker standard of proof.
However, it could be that the concern about intelligibility is not a concern about underlying reasons for thinking that a thing is but concern that we don't know what a thing is. But obviously if you have reasons to believe something exists you know what it is to the extent that you have reasons to believe it; the 'what' that contrasts with 'that' here has to do with our grounds for further inferences. In other words, "What would that even mean?" when applied to black holes would be the question, "On what basis could you say anything more about how this works?" or "What grounds does that provide for further inferences?" The reason that we associate this question with intelligibility is that the greater one's understanding of something, the more easily one can answer further questions. So, for instance, asked this question about black holes, the physicist would respond by talking about the most advanced theories about black holes currently available and the evidence about black holes that underlies them.
It's important to draw the distinction because which side of the distinction one is on at a given moment makes all the difference in the world. One of the most important distinctions to be drawn in any sort of inquiry is the distinction between problems that are problems for a theory and problems that are problems within it. The former put the theory into question; the latter are merely research problems. And making this distinction is essential to determining whether a theory is viable. The physics of black holes is certainly a viable set of theories; they are not made less viable by the fact that there are plenty of things about black holes that are not yet understood or that are purely speculative or that are based on extrapolation from other kinds of cases or that still involve a bit of guesswork. These are all points for further research, not problems for the theory. In talking about evolutionary theory, a biologist may say, "OK, but what does it mean to say that a population of reptile-like theropods eventually became a population of bird-like theropods?" If there were no immediate answer available, the biologist's response would not be, "Oh, then the theory must not be viable because the claim that one became the other is unintelligible." Rather, the biologist would respond by setting out to see what answer the question could be given, which would involve clearing up conceptual confusions, looking at evidence, etc. Such an inquiry might eventually uncover insuperable problems, i.e., problems that put the claim in question rather than simply raise questions for further research, but it might also end up justifying the theory as an excellent theory. A skeptic about evolution might ask, "What would it even mean?" in the same sense as the biologist; in which case he is simply trying to figure out where the theory leads. But he might also be trying to suggest with the question that the whole claim is just unintelligible; in which case the biologist doesn't have to respond by answering the research-problem question but simply by saying why the claim has enough evidential support and theoretical usefulness to merit provisional acceptance and further research. What the skeptic could not do is take the incompleteness of the answer to the research-problem to indicate that it is a fundamental problem for the theory itself; at least, asking "What could that even mean?" in this way would be selective obscurantism, or as we sometimes say more bluntly, playing stupid.
Viability of theories, in other words, is viability in terms of inquiry, and it makes a big difference to inquiry whether we are talking about what leads to a claim or what follows from it. Now, I think it's clear enough that Richard's argument as stated above fails to make this distinction properly. The 'attempts to make intelligible' are dealing with the research-problem question; but whether they fail or not is irrelevant to whether the theory is viable or intelligible, because they are all attempts to extend what we know about immaterial substances on the assumption that they exist. The only question that would be relevant to whether the theory is viable or intelligible would be whether its reasons for saying that thinking could not be a material activity are coherent and reasonable (even if ultimately wrong); but this is not what the argument considers. The argument then seems to founder on considerations of what constitutes viability.