 At this point the issue, then, divides into a theoretical question and an empirical one. The empirical question is whether or not there is evidence that forms of society exist only for as long as they advance productive power, and are replaced by revolution when they fail. Here, one must admit, the empirical record is patchy at best, and there appear to have been long periods of stagnation, even regression, when dysfunctional economic structures were not revolutionised.