The success of modern physics has consisted primarily in the analysis of physical reality into its fundamental components. But popular reports that physicists are close to arriving at a "theory of everything" are misleading: such a theory would still be limited to interpreting fundamental interactions and explaining why subatomic particles have the mass, charge, and other characteristics that they do. It would explain "everything" only in the sense that everything is made of particles. It could not predict the behavior of complicated systems like thunderstorms or human beings. Even if one could ascertain the locations and trajectories of all the subatomic particles in such a complicated system, no scientist could compute the behavior of all those particles for any significant time into the future.

The problem of prediction in complicated systems is especially intractable when the system in question is chaotic. The essential characteristic of chaotic systems is that they are "nonlinear." In linear systems, similar causes lead to similar effects, whereas in nonlinear systems the outcome of the process is so sensitive to initial conditions that a minuscule change at the beginning of the process results in a large difference in the end. Consider water flowing out of a faucet. At low velocities the water behaves in a linear fashion, flowing out in a predictable way. But if the faucet is opened wide enough, the water becomes highly turbulent and chaotic, spurting in unpredictable directions. The input is not dramatically different from the input that previously produced a linear result—the faucet is just opened wider—but the result is suddenly very different.

Between the extremes of simple predictability and chaos there can arise behavior that is partly linear and partly chaotic. The emerging science of complexity theory offers promise of explaining the behavior of systems that are on the edge of chaos, in the sense that they retain a degree of order while flirting with chaotic nonlinearity. For example, pockets of turbulence may appear in an otherwise smooth flow, such as when a flowing bathroom faucet suddenly starts rapping loudly. Much of the excitement about complexity theory arises because of the belief that living things are systems of this kind. Cell membranes, for example, are poised on the boundary between a solid and a liquid state and are thus somewhat unstable. Small changes in a cell's local chemistry can produce disproportionately large changes in the cell. While some of these changes are harmful to the cell and may even destroy it, most are compatible with its continued existence and some may even prove biologically useful. Thus, while the cell requires order for its survival, it also needs a degree of disorder to facilitate novelty—development of new structures and behaviors that enable it to cope with changing environmental conditions, and, thus, adapt and evolve.