Dave Gudeman at my old blog commented forcefully and eloquently:
I've always had difficulty with arguments like this:
It is not easy to understand how God could add causal input to the space-time system.
I'm aware that such arguments have a distinguished history, but I don't get it. Just because you don't understand how it works, you doubt that it is possible? But you don't really understand how anything works. Not matter, not energy, not beauty, not humor. Science pretends that it understands things, but if you trace their theories to the end, all they do is propose underlying mechanisms that suffer from the same opaque nature as what they are trying to explain.
Since you don't understand how any cause at all operates, what does it prove that you can't understand how God operates?
This deserves a careful response.
Suppose X is actual. It then follows that X is possible. (Ab esse ad posse valet illatio.) And this without prejudice to the fact, if it is a fact, that no one can explain how X is possible. So if I know that X is actual, then I know that X is possible despite my lack of understanding of 'how it works.' For example, if I know that I am free in the libertarian 'could have done otherwise' sense, then I know that libertarian freedom of the will is possible, and this knowledge cannot be overturned by my inability to explain how libertarian freedom of the will is possible in a deterministic universe, or in an indeterministic one, for that matter. The question, 'But how is it possible?' does not constitute a good reason to think it unactual assuming that I know that it is actual.
Now let's try the argument with 'miracles' plugged in for 'X.' If I know that miracles are actual, then I know that they are possible, and my inability to explain how they are possible has no tendency to show that miracles are not actual. So if I know that there has been a violation or suspension of a law of nature by a supernatural agent, in the form of a violation of the causal closure of the physical realm, then I know that it is possible that a supernatural agent has added causal input to the space-time system, and my inability to explain how this addition is possible has no tendency to show that it is not actual.
But note the difference between the two arguments. I know that I am free, but I don't know that miracles have occurred. (If you object that no one knows that he is libertarianly free, then I merely point out that my belief that I am libertarianly free, even if false in the final analysis, is far more credible than any belief in genuine (ontic) miracles. Another way to respond is by invoking a different value of 'X.' Surely, for some values of 'X' their actuality is known.)
The point is that I don't know that any miraculous events have occurred. I may believe it, and I may even have grounds for believing it. But of which putative miracle can I say that I know that it occurred in the sense in which I know that I am free? Since I don't know that miracles have occurred, it is reasonable to wonder whether they are even possible. So it is reasonable to inquire as to how they are possible. If it turns out that no one has a good explanation as to how they are possible, then this gives me reason to doubt whether they are possible.
Of course, my inability to explain how miracles are possible is not a proof that they are impossible. To think otherwise would be to succumb to a form of the ad ignorantiam fallacy. Nowhere have I argued that miracles are impossible. I have merely raised the questions as to what they are and how they are possible.
Dave asks, "Just because you don't understand how it works, you doubt that it is possible?" I answer: In the case of miracles, yes. In the case of other things, no. If I know that X is actual, then my lack of understanding as to 'how it works' cuts no ice, as explained above. But in the case of miracles, I don't know that they have occurred. It is therefore reasonable to inquire into how miracles are possible. If no one can explain how they are possible, not even in a rough and schematic way, then it seems I should suspend belief in the possibility of miracles. Otherwise I believe in something for which I have no rational warrant.
Dave writes, "But you don't really understand how anything works." False. Surely we (or some of us) understand how our artifacts work. I understand how the engine in my car works because I understand all its components and how they are connected together. Of course I don't understand all the underlying chemistry and physics. But to understand how an artifact works, it is not necessary to understand all the underlying properties of matter, or why there is matter at all, or what causation is, etc.
To sum up. What is actual is possible. So if I know that X is actual, then I know that X is possible regardless of whether or not I can explain how X is possible. But if I don't know that X is actual, and I have good reason to doubt whether X is possible, because I cannot explain how X is possible given what I do know, then I have good reason to doubt whether X is actual.