"Why am I me?"

This question was asked by my eight-year-old grandson George. In eight letters it summarizes the conundrum of personal existence in an impersonal universe. How does it happen that a couple of liters of grey matter organizes itself into the unique stream of self-awareness that calls itself George? If we could answer this question, we would be on the way toward an understanding of brain structure and function at a deep level. We would probably have in our hands the key to a more rational and discriminating treatment of mental illnesses. We might also have the key to the design of a genuine artificial intelligence.

Every human being must have asked this question in one way or another. For most of us, the question expresses only a general philosophical curiosity about our place in the order of nature. But for George the question has a more specific technical meaning. He has an identical twin brother Donald, and he understands the distinction between monozygotic and fraternal twins. He knows that he and Donald not only have the same genes but also have the same environment and upbringing. When George asks the question, he is asking how it happens that two people with identical genes and identical nurture are nevertheless different. What are the non-genetic and non-environmental processes in the brain that cause George to be George and cause Donald to be Donald? If we could answer this question, we would have a powerful new tool for the investigation of cognitive development. The conventional wisdom says that mental differences between George and Donald arise from local randomness of neural connections, undetermined either by genes or by sensory input. But to say that the connections are random only means that we do not yet understand how they came about.

Freeman Dyson is professor of physics at the Institute for Advanced Study and author of The Sun, the Genome, and the Internet.