I sometimes find myself talking about my dad in the past tense.

He isn’t dead—in fact, he’s in good physical health (you can even see him dancing in this video).

But because he has Alzheimer’s Disease, I’ve had to watch him slowly become a different man than the one I grew up with. Reconciling the two people—the one he was and the one he is now—is incredibly difficult.

To fully grasp how much has changed, you have to know a little bit about my dad. Bob Blackwell was one of the smartest people I have ever met. This isn’t just idol worshipping my father (and he would be the first to disagree with my statement), it’s really true.

You could ask him any question on a range of topics and he would at first tell you he wasn’t the person to ask—before then launching into an explanation that was detailed, insightful and impressive. As a kid, this could drive me a bit bonkers. Ask him a question and you were going to need at least 20 minutes for the full answer.

He also had a stunning ability to recall information. Even until relatively recently, he could tell you state-by-state political poll numbers and the latest baseball stats. I seldom saw him lose a game of Trivial Pursuit. There were few subjects he couldn’t opine intelligently on.

Heck, he even knew how Darth Vader ended up in a mechanical suit—a good 20 years before George Lucas told the world in 2005.

It wasn’t just me that was impressed. Hanging on my parent’s wall is a letter from former President George H.W. Bush, who called my dad “one of the brightest men I’ve ever worked with.” As the CIA’s top expert on theSoviet Unionat the end of the Cold War, my dad briefed several presidents and impressed countless government officials. In a war-game scenario in the late 1980s, he fought—and defeated—former Rep. Newt Gingrich who played the president of the United States while my dad controlled the Soviets.

To see him today, however, it’s hard to remember the man my father was. Alzheimer’s has largely robbed him of his ability to keep up with politics, foreign affairs and even his beloved Atlanta Braves. Formerly a voracious reader, my dad has been stuck at the same point in a paperback book for the past several years. I bought him the book—a historical fiction by Jeff Shaara—about five years ago, and my mom had to replace it finally because it was so dog-eared from his repeated attempts to read it.

He sometimes forgets who my mom is and has concluded I’m famous because I’ve published a book (sadly, this is not true). In a conversation, he has trouble making an understandable point (although sometimes if you stick with him, you will find he is making a good one). Two years ago, he knew he had Alzheimer’s and was determined to fight it with his entire being. Asked last week by the local TV news crew if he had the disease, however, he sincerely claimed he didn’t have it.

In so many ways, my dad is a completely different person than he was.

And yet, I can still see him in there. For all his memory loss, his personality is still much the same (something that is not always the case with those who have Alzheimer’s). As the video above showed, he still likes to dance. He still enjoys hiking, plays a mean game of tennis on the Nintendo Wii, and is outgoing and friendly (my dad was the guy who would strike up a conversation with a stranger on a ski lift).

Even though he doesn’t always know who I am, when he does, he still tells me he is proud of me. He plays with my kids—in the pool during the summer or just two weeks ago, knocking around a basketball with my son. He remains devoted to his mother, my 98-year-old grandmother who was diagnosed with Alzheimer’s when she was 74 (my dad, in contrast, was diagnosed when he was 64).

It is so easy for me to be angry about how Alzheimer’s has changed my father, that I can lose sight of what I still have. Alzheimer’s is a devastating disease, not only for the person who has it, but for their spouse, family and friends. It robs a person one step at a time, letting those on the sidelines watch the slow, painful progression.

But one thing it has taught me is this: Be grateful for what you have, as long as you have it. Life is meant to be lived in the present tense. I love my dad—the man he was and the man he is.

(Rob Blackwell is the author of the supernatural thriller,  A SOUL TO STEAL).