Writer David Shenk's book The Forgetting has been called "the definitive work on Alzheimer's"-the story of the disease, its impact on patients and families, and the search for a cure. A few days after the death of Ronald Reagan from Alzheimer's, David Shenk spoke to Beliefnet's executive editor Elizabeth Sams about the trajectory of the president's illness and the spiritual meaning of the disease.

Everyone is of course wondering what impact his Alzheimer's may or may not have had on Reagan's time as president. Given that he died in 2004-and was diagnosed in 1994-what would our general knowledge of the disease say about that?

The first thing to say is that Reagan did not have Alzheimer's as president. It's more complex than that, but if you have to answer "Did he or did he not," he definitely did not have Alzheimer's, despite what some of his critics want to believe.

Reagan was very aware of senile dementia. Two members of his family had had it and he had been asked about it as he was running for president. He joked about it quite a bit as President-and I think he intuitively knew that if he lived long enough, he was headed in that direction.

He definitely joked about it while he was president. I have no doubt he was having some kind of what I would characterize as annoying memory issues-probably in his second term. And there's the suspicion that "Oh, they didn't want to admit it." And I'm sure they wouldn't have wanted to admit it if it had been there. But I think it's pretty clear from looking at the evidence that he could not have been diagnosed with Alzheimer's at that point. I don't think he had any real disorientation. I don't think his memory problems were anything really out of the ordinary for someone his age.

We all have memory problems as we get older. We also, more to the point, all have memory complaints as we mature, as we have higher and higher expectations of what we want to remember in this world. As we get older we meet more people, we learn more things, we want to remember it all. And it's not that the brain doesn't have the capacity to do that-it does. But the brain is actually built to help you forget a lot of the minute details so you can form executive summaries of things. That's really what intelligence comes down to-the ability to take a step back and not remember every tree and leaf in the forest but to recognize the shape of the forest-to be able to characterize it.

After [Reagan] left the Presidency I think it's clear that he had progressively more difficulty with his memory and started to have issues of disorientation in the early '90s. And I think by the time he was diagnosed [in 1994] there wasn't any doubt. He probably could have been diagnosed a little earlier.

Alzheimer's is a very predictable disease. It can vary in how long it actually takes to completely overcome you. It can range from say five to even 15 or 20 years, but Reagan's ten years from diagnosis to death is right there in the average.

In all your conversations and research for the book, did you get any insight into how Alzheimer's affects spirituality?

I think Alzheimer's has an enormous effect on our spiritual lives in that those of us who come into contact with it-as friends, family members, doctors, scientists, or other level of observer-because you're seeing this excruciating loss going on in very slow motion. You're seeing these "shaves" of a person-like the layers of an onion-slowly peeling away. And it's hard to watch. But there's also something very profound and meaningful in the loss-seeing it so up close. Also in the idea of going through stages of mourning in such an incredibly slow way-which is very rare. I don't think there's any other disease quite like this-where people mourn, are forced to mourn the loss [over so long.] I mean, you have slow diseases, but this one is not just killing you slowly but it's taking you away slowly-taking the person that we recognize away from us. It's not like you're here one day and then gone the next. You are here, and then a little bit less of you is here, and then a little bit less-and that goes on for ten or 15 years.

I've heard some observers say that on one level Nancy Reagan must be relieved, and she's not going through the classic mourning process that you would see if her spouse had a heart attack. Of course there's probably a lot of grieving going on now and will for some time. But a lot of that loss has already happened and happened so slowly that it really does shake you and you're forced to come up with some kind of spiritual meaning. Whether that means a return to your religious roots or some new idea of religion or whether it's not really religious per se. I know for me the experience of being in this [Alzheimer's] world has just been incredibly spiritual. It hasn't sent me to my religious roots, but it has been very, very meaningful in a way that I tried to write about, but it's difficult.