THERE IS AN "I" IN "ME":

The 19th-century philosopher William James proposed that the self could be split into two parts. The first was an "I" that physically perceives and experiences the world, and the second was a "me" that encompasses a mental narrative about oneself, based on one's past experiences. Neuroscientists equipped with high-tech tool kits have begun to achieve some success in the long-running search to find the brain areas responsible for creating these two aspects of the self.

The discovery of "me" came first. The default-mode network, a term coined by neurologist Marcus Raichle in 2001, has emerged as a key player in the "me" aspect of the self. This collection of brain areas is active when a person is not focused on a task, and researchers have found that it plays an important role in processing self-referential thoughts. "[This network] has kind of been baptized as the center for the sense of self," says Josef Parvizi, a neurologist and a professor at Stanford University who researches the self.

The "I," in contrast, has been harder to pin down--at least until very recently. The awareness we have that we inhabit a body (call it an essential "I-ness") forms a bridge that constantly switches back and forth between a conscious and unconscious state of mind. Suppose you're sitting at the kitchen table or standing waiting for a train. Unless you're in pain, you have no moment-by-moment awareness of your hand, your shin, your big toe or even your body as a whole. But as soon as you think of any of these spots, you can feel their presence immediately. "I-ness" is that feeling that you indeed occupy your own body.

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