But over the past millennium, many of us have undergone a profound shift. We've gradually replaced our internal memory with what psychologists refer to as external memory, a vast superstructure of technological crutches that we've invented so that we don't have to store information in our brains. We've gone, you might say, from remembering everything to remembering awfully little. We have photographs to record our experiences, calendars to keep track of our schedules, books (and now the Internet) to store our collective knowledge, and Post-it notes for our scribbles. What have the implications of this outsourcing of memory been for ourselves and for our society? Has something been lost?

To supplement the memories in her mind, AJ also stores a trove of external memories. In addition to the detailed diary she's kept since childhood, she has a library of close to a thousand videotapes copied off TV, a trunk full of radio recordings, and a "research library" consisting of 50 notebooks filled with facts she's found on the Internet that relate to events in her memory. "I just want to keep it all," she says.

Preserving her past has become the central compulsion of AJ's life. "When I'm blow-drying my hair in the morning, I'll think of whatever day it is. And to pass the time, I'll just run through that day in my head over the last 20-something years—like flipping through a Rolodex."

AJ traces the origins of her unusual memory to a move from New Jersey to California that her family made when she was just eight years old. Life in New Jersey had been comfortable and familiar, and California was foreign and strange. It was the first time she understood that growing up and moving on necessarily meant forgetting and leaving behind. "Because I hate change so much, after that it was like I wanted to be able to capture everything. Because I know, eventually, nothing will ever be the same," she says.

K. Anders Ericsson, a professor of psychology at Florida State University, believes that at bottom, AJ might not be all that different from the rest of us. After the initial announcement of AJ's condition in the journal Neurocase, Ericsson suggested that what needs to be explained about AJ is not some extraordinary, unprecedented innate memory but rather her extraordinary obsession with her past. People always remember things that are important to them. Baseball fanatics often have an encyclopedic knowledge for statistics, chess masters often remember tricky gambits that took place years ago, actors often remember scripts long after performing them. Everyone has got a memory for something. Ericsson believes that if anyone cared about holding on to the past as much as AJ does, the feat of memorizing one's life would be well within reach.