Artful, how does something non-physical/immaterial [sic] interact with the physical/material? Wouldn't such interaction be subject to scientific inquiry? Why has no such interaction been observed?

This is a very good question riandouglas. This is one of the areas that I feel I need to explore a bit more. There are different views within the Christian community. Suffice it to say that yes, the interaction will be open to scientific enquiry. Neuroimaging can tell neuroscientists what is going on in a person's brain when they are engaged in any "soulish" activity: listening to music, enjoying a conversation, reasoning, making a moral choice, having a spritual experience and so forth. Our body and the "self" that inhabits it are fully and completely integrated. But that does not mean that what the person is experiencing is nothing but the pattern of electical discharges that show up on the neuroimage. The neurological changes that occur are open to scientific scrutiny, but not the "meaning" of the experience in the consciousness of the individual. If our ideas and reasonings were fully explicable in terms of these physiological phenomena, then Lewis' point was precisely that the content of our beliefs would be invalidated.