“How does individual behavior aggregate to collective behavior? As simply as it can be asked, this is one of the most fundamental and pervasive questions in all of science. A human brain, for example, is in one sense a trillion neurons connected in a big electrochemical lump. But to each of us who has one, a brain is clearly much more, exhibiting properties like consciousness, memory, and personality, whose nature cannot be explained simply in terms of aggregations of neurons. What makes the problem hard, and what makes complex systems complex, is that the parts making up the whole don’t sum up in any simple fashion. Rather, they interact with each other, and in interacting, even quite simple components can generate bewildering behavior.”