 Sitting in the background for all of these discussions is nonlinearity. Chaos only exists in nonlinear systems (at least for classical macroscopic systems; see sec. 6 for subtitles regarding quantum chaos). Nonlinearity appears to be a necessary condition for the stretching and folding mechanisms, so would seem to be a necessary condition for chaotic behavior. However, there is an alternative way to characterize the systems in which such stretching and folding takes place: nonseparability. 