My father and I are saying goodbye at a small airport in southern Africa. He and close friends of his have joined me in one of my favorite places, Botswana's Okavango Delta. We've always been close, but for some reason he seems especially emotional as I put him on the plane. Tears well in his eyes as he says how much he loves me and hopes we'll see more of each other. I assure him that I'll be home soon. He smiles and climbs into the plane.

Immediately I call my mother and sister and tell them that something is not right. During our safari he became easily confused. He drifted off in conversations. He seemed disengaged. One evening as we talked, Dad—a world traveler and geography whiz—couldn't remember the name of the Swiss village he and my mother stayed in at least a dozen times.

My mother takes him to a neurologist for testing. The diagnosis is dementia, most likely Alzheimer's. Dad remains cheerful and positive. As often happens in these cases, my mother is the one who struggles with despair. Shortly thereafter, she is diagnosed with cancer. Six months later, she is gone.

My sister and I face the toughest decision of our lives: How to give our father the care he deserves? We find an excellent facility, three miles from my sister's home, that specializes in caring for those with dementia. At first he resists, then settles in. When I call, my father tells me he's buying a new yellow Mustang, and that he and my mother are driving over to visit this afternoon. It breaks my heart to hear his gentle voice making plans that will never happen, but then I think that if he is happy living in an imaginary world with his beloved wife, perhaps memory loss isn't such a bad thing. I accept his illness and cherish every moment with him.

Memory, perishable and enduring, is the brain's archive. It is a marvel of neuronic circuitry, as Joshua Foer explains in this month's cover story. Its loss can be cruel, but remember this: It is through memory that we hold on to those we love.