"A world without memory is a world of the present," Alan Lightman wrote in Einstein's Dreams. "The past exists only in books, in documents. In order to know himself, each person carries his own Book of Life, which is filled with the history of his life...Without his Book of Life, a person is a snapshot, a two-dimensional image, a ghost."

Most people would probably agree with Lightman. Most people think that our self -knowledge exists only through the memories we have amassed of our selves. Am I a kind person? Am I gloomy? To answer these sorts of questions, most people would think you have to open up some internal Book of Life. And most people, according to new research, are wrong.

Neuroscientists would call Lightman's Book of Life episodic memory. The human brain has a widespread system of neurons that store away explicit memories of events, which we can recall and describe to others. Some forms of amnesia destroy episodic memories, and sometimes even destroy the capacity to form new ones. In 2002, Stan B. Klein of the University of California at Santa Barbara and his colleagues reported a study they made of an amnesiac known as D.B. D.B. was 75 years old when he had a heart attack and lost his pulse. His heart began to beat after a few minutes, and he left the hospital after a few weeks. But he had suffered brain damage that left him unable to bring to mind anything had done or experienced before the heart attack. Klein then tested D.B.'s self-knowledge. He gave D.B. a list of 60 traits and asked him whether they applied to him not at all, somewhat, quite a bit, or definitely. Then he gave the same questionnaire to D.B.'s daughter, and asked her to use it to describe her fater. D.B.'s choices significantly correlated with his daughter's. D.B.'s Book of Life was locked shut, and yet he still knew himself.

A few other amnesiacs have shown a similar level of self-knowledge, but it's hard to draw too many lessons from them about how normal brains work. So recently Matthew Lieberman of UCLA and his colleagues carried out a brain-scanning study. They wanted to see if they could find different networks in the brain that make self-knowledge possible. They also wanted to see if these networks functioned under different circumstances--for example, when thinking about ourselves in very familiar contexts and unfamiliar ones.

They picked two groups of people to test: soccer players and improv actors. They then came up with a list of words that would apply to each group. (Soccer players: athletic, strong, swift; actors: performer, dramatic, etc.) They also came up with a longer list of words that applied specifically to neither (messy, reliable, etc.). Then they had all the subjects get into an fMRI scanner, look at each word, and decide whether it applied to themselves or not.

The volunteers' brains worked differently in response to different words. Soccer-related words tended to activate a distinctive network in the brains of soccer players, the same one that actor-related words switched on in actors. When they were shown words related to the other group, a different network became active. And, as Lieberman and his colleagues report in an upcoming issue of the Journal of Personality and Social Psychology, it just so happens that they had predicted precisely which two networks would show up in their scans.

When people were presented with unfamiliar words, they activated a network Lieberman calls the Reflective system (or C system for short). The Reflective system taps into parts of the brain already known to retrieve episodic memories. It also includes regions that can consciously hold pieces of information in mind. When we are in new circumstances, our sense of our self depends on thinking explicitly about our experiences.

But Lieberman argues that over time, another system takes over. He calls this one the Reflexive system (or X system). This circuit does not include regions involved in episodic memories, such as the hippocampus. Instead, it is an intuition network, tapping into regions that produce quick emotional responses based not on explicit reasoning but on statistical associations. (The picture I show here is a figure from the paper, with the X and C systems mapped out.)

The Reflexive system is slow to form its self-knowledge, because it needs a lot of experiences to form these associations. But it becomes very powerful once it takes shape. A soccer player knows whether he is athletic, strong, or swift without having to open up the Book of Life. He just feels it in his bones. He doesn't feel in his bones whether he is a performer, or dramatic, and so on. Instead, he has to think explicity about his experiences. Now D.B.'s accurate self-knowledge makes sense. His brain damage wiped out his Reflective system, but not his Reflexive system.