In the rejection of nineteenth-century social science, world systems analysis necessarily rejects its reigning faith, the belief in inevitable progress. I believe that a viable alternative model of change is that of nonlinear processes which eventually reach bifurcation points, whereupon slight fluctuations have large consequences (as opposed to determinate equilibria in which large fluctuations have small consequences). This is the model Prigogine has suggested for all complex systems (“order through chaos”)-and the most complex of all known systems is an historical social system. Even for such simple systems as physical systems, the key variable becomes time, reconceptualizing reality as involving stochastic and irreversible processes, within which deterministic , reversible processes constitute a limited, special case. A fortiori for complex historical systems.